🔭Data Science: Underfitting (Just the Quotes)

"A smaller model with fewer covariates has two advantages: it might give better predictions than a big model and it is more parsimonious (simpler). Generally, as you add more variables to a regression, the bias of the predictions decreases and the variance increases. Too few covariates yields high bias; this called underfitting. Too many covariates yields high variance; this called overfitting. Good predictions result from achieving a good balance between bias and variance. […] finding a good model involves trading of fit and complexity." (Larry A Wasserman, "All of Statistics: A concise course in statistical inference", 2004)

"When generating trees, it is usually optimal to grow a larger tree than is justifiable and then prune it back. The main reason this works well is that stop splitting rules do not look far enough forward. That is, stop splitting rules tend to underfit, meaning that even if a rule stops at a split for which the next candidate splits give little improvement, it may be that splitting them one layer further will give a large improvement in accuracy." (Bertrand Clarke et al, "Principles and Theory for Data Mining and Machine Learning", 2009)

"Briefly speaking, to solve a Machine Learning problem means you optimize a model to fit all the data from your training set, and then you use the model to predict the results you want. Therefore, evaluating a model need to see how well it can be used to predict the data out of the training set. Usually there are three types of the models: underfitting, fair and overfitting model [...]. If we want to predict a value, both (a) and (c) in this figure cannot work well. The underfitting model does not capture the structure of the problem at all, and we say it has high bias. The overfitting model tries to fit every sample in the training set and it did it, but we say it is of high variance. In other words, it fails to generalize new data." (Shudong Hao, "A Beginner’s Tutorial for Machine Learning Beginners", 2014)

"There are two kinds of mistakes that an inappropriate inductive bias can lead to: underfitting and overfitting. Underfitting occurs when the prediction model selected by the algorithm is too simplistic to represent the underlying relationship in the dataset between the descriptive features and the target feature. Overfitting, by contrast, occurs when the prediction model selected by the algorithm is so complex that the model fits to the dataset too closely and becomes sensitive to noise in the data."(John D Kelleher et al, "Fundamentals of Machine Learning for Predictive Data Analytics: Algorithms, Worked Examples, and Case Studies", 2015)

"Underfitting is when a model doesn’t take into account enough information to accurately model real life. For example, if we observed only two points on an exponential curve, we would probably assert that there is a linear relationship there. But there may not be a pattern, because there are only two points to reference. [...] It seems that the best way to mitigate underfitting a model is to give it more information, but this actually can be a problem as well. More data can mean more noise and more problems. Using too much data and too complex of a model will yield something that works for that particular data set and nothing else." (Matthew Kirk, "Thoughtful Machine Learning", 2015)

"Bias is error from incorrect assumptions built into the model, such as restricting an interpolating function to be linear instead of a higher-order curve. [...] Errors of bias produce underfit models. They do not fit the training data as tightly as possible, were they allowed the freedom to do so. In popular discourse, I associate the word 'bias' with prejudice, and the correspondence is fairly apt: an apriori assumption that one group is inferior to another will result in less accurate predictions than an unbiased one. Models that perform lousy on both training and testing data are underfit." (Steven S Skiena, "The Data Science Design Manual", 2017)

"Bias occurs normally when the model is underfitted and has failed to learn enough from the training data. It is the difference between the mean of the probability distribution and the actual correct value. Hence, the accuracy of the model is different for different data sets (test and training sets). To reduce the bias error, data scientists repeat the model-building process by resampling the data to obtain better prediction values." (Umesh R Hodeghatta & Umesha Nayak, "Business Analytics Using R: A Practical Approach", 2017)

"High-bias models typically produce simpler models that do not overfit and in those cases the danger is that of underfitting. Models with low-bias are typically more complex and that complexity enables us to represent the training data in a more accurate way. The danger here is that the flexibility provided by higher complexity may end up representing not only a relationship in the data but also the noise. Another way of portraying the bias-variance trade-off is in terms of complexity v simplicity." (Jesús Rogel-Salazar, "Data Science and Analytics with Python", 2017) 

"If either bias or variance is high, the model can be very far off from reality. In general, there is a trade-off between bias and variance. The goal of any machine-learning algorithm is to achieve low bias and low variance such that it gives good prediction performance. In reality, because of so many other hidden parameters in the model, it is hard to calculate the real bias and variance error. Nevertheless, the bias and variance provide a measure to understand the behavior of the machine-learning algorithm so that the model model can be adjusted to provide good prediction performance." (Umesh R Hodeghatta & Umesha Nayak, "Business Analytics Using R: A Practical Approach", 2017)

"Overfitting and underfitting are two important factors that could impact the performance of machine-learning models. Overfitting occurs when the model performs well with training data and poorly with test data. Underfitting occurs when the model is so simple that it performs poorly with both training and test data. [...]  When the model does not capture and fit the data, it results in poor performance. We call this underfitting. Underfitting is the result of a poor model that typically does not perform well for any data." (Umesh R Hodeghatta & Umesha Nayak, "Business Analytics Using R: A Practical Approach", 2017)

"Overfitting refers to the phenomenon where a model is highly fitted on a dataset. This generalization thus deprives the model from making highly accurate predictions about unseen data. [...] Underfitting is a phenomenon where the model is not trained with high precision on data at hand. The treatment of underfitting is subject to bias and variance. A model will have a high bias if both train and test errors are high [...] If a model has a high bias type underfitting, then the remedy can be to increase the model complexity, and if a model is suffering from high variance type underfitting, then the cure can be to bring in more data or otherwise make the model less complex." (Danish Haroon, "Python Machine Learning Case Studies", 2017)

"The tension between bias and variance, simplicity and complexity, or underfitting and overfitting is an area in the data science and analytics process that can be closer to a craft than a fixed rule. The main challenge is that not only is each dataset different, but also there are data points that we have not yet seen at the moment of constructing the model. Instead, we are interested in building a strategy that enables us to tell something about data from the sample used in building the model." (Jesús Rogel-Salazar, "Data Science and Analytics with Python", 2017)

"Even though a natural way of avoiding overfitting is to simply build smaller networks (with fewer units and parameters), it has often been observed that it is better to build large networks and then regularize them in order to avoid overfitting. This is because large networks retain the option of building a more complex model if it is truly warranted. At the same time, the regularization process can smooth out the random artifacts that are not supported by sufficient data. By using this approach, we are giving the model the choice to decide what complexity it needs, rather than making a rigid decision for the model up front (which might even underfit the data)." (Charu C Aggarwal, "Neural Networks and Deep Learning: A Textbook", 2018)

"One of the most common problems that you will encounter when training deep neural networks will be overfitting. What can happen is that your network may, owing to its flexibility, learn patterns that are due to noise, errors, or simply wrong data. [...] The essence of overfitting is to have unknowingly extracted some of the residual variation (i.e., the noise) as if that variation represented the underlying model structure. The opposite is called underfitting - when the model cannot capture the structure of the data." (Umberto Michelucci, "Applied Deep Learning: A Case-Based Approach to Understanding Deep Neural Networks", 2018)

"The high generalization error in a neural network may be caused by several reasons. First, the data itself might have a lot of noise, in which case there is little one can do in order to improve accuracy. Second, neural networks are hard to train, and the large error might be caused by the poor convergence behavior of the algorithm. The error might also be caused by high bias, which is referred to as underfitting. Finally, overfitting (i.e., high variance) may cause a large part of the generalization error. In most cases, the error is a combination of more than one of these different factors." (Charu C Aggarwal, "Neural Networks and Deep Learning: A Textbook", 2018)

"The trick is to walk the line between underfitting and overfitting. An underfit model has low variance, generally making the same predictions every time, but with extremely high bias, because the model deviates from the correct answer by a significant amount. Underfitting is symptomatic of not having enough data points, or not training a complex enough model. An overfit model, on the other hand, has memorized the training data and is completely accurate on data it has seen before, but varies widely on unseen data. Neither an overfit nor underfit model is generalizable - that is, able to make meaningful predictions on unseen data." (Benjamin Bengfort et al, "Applied Text Analysis with Python: Enabling Language-Aware Data Products with Machine Learning", 2018)

"Any fool can fit a statistical model, given the data and some software. The real challenge is to decide whether it actually fits the data adequately. It might be the best that can be obtained, but still not good enough to use." (Robert Grant, "Data Visualization: Charts, Maps and Interactive Graphics", 2019)

🔭Data Science: Statistical Modeling (Just the Quotes)

"The most widely used mathematical tools in the social sciences are statistical, and the prevalence of statistical methods has given rise to theories so abstract and so hugely complicated that they seem a discipline in themselves, divorced from the world outside learned journals. Statistical theories usually assume that the behavior of large numbers of people is a smooth, average 'summing-up' of behavior over a long period of time. It is difficult for them to take into account the sudden, critical points of important qualitative change. The statistical approach leads to models that emphasize the quantitative conditions needed for equilibrium - a balance of wages and prices, say, or of imports and exports. These models are ill suited to describe qualitative change and social discontinuity, and it is here that catastrophe theory may be especially helpful." (Alexander Woodcock & Monte Davis, "Catastrophe Theory", 1978)

"Statistical models for data are never true. The question whether a model is true is irrelevant. A more appropriate question is whether we obtain the correct scientific conclusion if we pretend that the process under study behaves according to a particular statistical model." (Scott Zeger, "Statistical reasoning in epidemiology", American Journal of Epidemiology, 1991)

"[…] it does not seem helpful just to say that all models are wrong. The very word model implies simplification and idealization. The idea that complex physical, biological or sociological systems can be exactly described by a few formulae is patently absurd. The construction of idealized representations that capture important stable aspects of such systems is, however, a vital part of general scientific analysis and statistical models, especially substantive ones, do not seem essentially different from other kinds of model." (Sir David Cox, "Comment on ‘Model uncertainty, data mining and statistical inference’", Journal of the Royal Statistical Society, Series A 158, 1995)

"Building statistical models is just like this. You take a real situation with real data, messy as this is, and build a model that works to explain the behavior of real data." (Martha Stocking, New York Times, 2000)

"The role of graphs in probabilistic and statistical modeling is threefold: (1) to provide convenient means of expressing substantive assumptions; (2) to facilitate economical representation of joint probability functions; and (3) to facilitate efficient inferences from observations." (Judea Pearl, "Causality: Models, Reasoning, and Inference", 2000)

"It is impossible to construct a model that provides an entirely accurate picture of network behavior. Statistical models are almost always based on idealized assumptions, such as independent and identically distributed (i.i.d.) interarrival times, and it is often difficult to capture features such as machine breakdowns, disconnected links, scheduled repairs, or uncertainty in processing rates." (Sean Meyn, "Control Techniques for Complex Networks", 2008)

"Statistical cognition is concerned with obtaining cognitive evidence about various statistical techniques and ways to present data. It’s certainly important to choose an appropriate statistical model, use the correct formulas, and carry out accurate calculations. It’s also important, however, to focus on understanding, and to consider statistics as communication between researchers and readers." (Geoff Cumming, "Understanding the New Statistics", 2012)

"Statistical models in the social sciences rely on correlations, generally not causes, of our behavior. It is inevitable that such models of reality do not capture reality well. This explains the excess of false positives and false negatives." (Kaiser Fung, "Numbersense: How To Use Big Data To Your Advantage", 2013

"In general, when building statistical models, we must not forget that the aim is to understand something about the real world. Or predict, choose an action, make a decision, summarize evidence, and so on, but always about the real world, not an abstract mathematical world: our models are not the reality - a point well made by George Box in his oft-cited remark that "all models are wrong, but some are useful". (David Hand, "Wonderful examples, but let's not close our eyes", Statistical Science 29, 2014)

"Once a model has been fitted to the data, the deviations from the model are the residuals. If the model is appropriate, then the residuals mimic the true errors. Examination of the residuals often provides clues about departures from the modeling assumptions. Lack of fit - if there is curvature in the residuals, plotted versus the fitted values, this suggests there may be whole regions where the model overestimates the data and other whole regions where the model underestimates the data. This would suggest that the current model is too simple relative to some better model." (DeWayne R Derryberry, "Basic data analysis for time series with R", 2014)

"Prediction about the future assumes that the statistical model will continue to fit future data. There are several reasons this is often implausible, but it also seems clear that the model will often degenerate slowly in quality, so that the model will fit data only a few periods in the future almost as well as the data used to fit the model. To some degree, the reliability of extrapolation into the future involves subject-matter expertise." (DeWayne R Derryberry, "Basic data analysis for time series with R", 2014)

"The random element in most data analysis is assumed to be white noise - normal errors independent of each other. In a time series, the errors are often linked so that independence cannot be assumed (the last examples). Modeling the nature of this dependence is the key to time series." (DeWayne R Derryberry, "Basic data analysis for time series with R", 2014)

"A statistical model is a relatively simple approximation to account for complex phenomena that generate data. A statistical model consists of one or more equations involving both random variables and parameters. The random variables have stated or assumed distributions. The parameters are unknown fixed quantities. The random components of statistical models account for the inherent variability in most observed phenomena." (Richard M Heiberger & Burt Holland, "Statistics Concepts", 2015)

"An oft-repeated rule of thumb in any sort of statistical model fitting is 'you can't fit a model with more parameters than data points'. This idea appears to be as wide-spread as it is incorrect. On the contrary, if you construct your models carefully, you can fit models with more parameters than datapoints [...]. A model with more parameters than datapoints is known as an under-determined system, and it's a common misperception that such a model cannot be solved in any circumstance. [...] this misconception, which I like to call the 'model complexity myth' [...] is not true in general, it is true in the specific case of simple linear models, which perhaps explains why the myth is so pervasive." (Jake Vanderplas, "The Model Complexity Myth", 2015) [source]

"Machine learning takes many different forms and goes by many different names: pattern recognition, statistical modeling, data mining, knowledge discovery, predictive analytics, data science, adaptive systems, self-organizing systems, and more. Each of these is used by different communities and has different associations. Some have a long half-life, some less so." (Pedro Domingos, "The Master Algorithm", 2015)

"In machine learning, knowledge is often in the form of statistical models, because most knowledge is statistical [...] Machine learning is a kind of knowledge pump: we can use it to extract a lot of knowledge from data, but first we have to prime the pump." (Pedro Domingos, "The Master Algorithm", 2015)

"One final warning about the use of statistical models (whether linear or otherwise): The estimated model describes the structure of the data that have been observed. It is unwise to extend this model very far beyond the observed data." (David S Salsburg, "Errors, Blunders, and Lies: How to Tell the Difference", 2017)

"The central limit conjecture states that most errors are the result of many small errors and, as such, have a normal distribution. The assumption of a normal distribution for error has many advantages and has often been made in applications of statistical models." (David S Salsburg, "Errors, Blunders, and Lies: How to Tell the Difference", 2017)

"When we use algebraic notation in statistical models, the problem becomes more complicated because we cannot 'observe' a probability and know its exact number. We can only estimate probabilities on the basis of observations." (David S Salsburg, "Errors, Blunders, and Lies: How to Tell the Difference", 2017)

"Any fool can fit a statistical model, given the data and some software. The real challenge is to decide whether it actually fits the data adequately. It might be the best that can be obtained, but still not good enough to use." (Robert Grant, "Data Visualization: Charts, Maps and Interactive Graphics", 2019)

"Statistical models have two main components. First, a mathematical formula that expresses a deterministic, predictable component, for example the fitted straight line that enables us to make a prediction [...]. But the deterministic part of a model is not going to be a perfect representation of the observed world [...] and the difference between what the model predicts, and what actually happens, is the second component of a model and is known as the residual error - although it is important to remember that in statistical modelling, ‘error’ does not refer to a mistake, but the inevitable inability of a model to exactly represent what we observe." (David Spiegelhalter, "The Art of Statistics: Learning from Data", 2019)

🔭Data Science: Principles (Just the Quotes)

"In all disciplines in which there is systematic knowledge of things with principles, causes, or elements, it arises from a grasp of those: we think we have knowledge of a thing when we have found its primary causes and principles, and followed it back to its elements." (Aristotle, "Physics", cca. 350 BC)

"[…] the least initial deviation from the truth is multiplied later a thousand-fold. Admit, for instance, the existence of a minimum magnitude, and you will find that the minimum which you have introduced, small as it is, causes the greatest truths of mathematics to totter. The reason is that a principle is great rather in power than in extent; hence that which was small at the start turns out a giant at the end." (St. Thomas Aquinas, "De Ente et Essentia", cca. 1252)

"It is superfluous to suppose that what can be accounted for by a few principles has been produced by many." (Thomas Aquinas, "Summa Theologica", cca. 1266-1273)

"Reality cannot be found except in One single source, because of the interconnection of all things with one another. […] It is a good thing to proceed in order and to establish propositions (principles). This is the way to gain ground and to progress with certainty." (Gottfried Leibniz, 1670)

"Every science has for its basis a system of principles as fixed and unalterable as those by which the universe is regulated and governed. Man cannot make principles; he can only discover them." (Thomas Paine, "The Age of Reason", 1794)

"A maxim is a conclusion upon observation of matters of fact, and is merely speculative; a ‘principle’ carries knowledge within itself, and is prospective." (Samuel T Coleridge, "The Table Talk and Omniana of Samuel Taylor Coleridge", 1831)

"The function of theory is to put all this in systematic order, clearly and comprehensively, and to trace each action to an adequate, compelling cause. […] Theory should cast a steady light on all phenomena so that we can more easily recognize and eliminate the weeds that always spring from ignorance; it should show how one thing is related to another, and keep the important and the unimportant separate. If concepts combine of their own accord to form that nucleus of truth we call a principle, if they spontaneously compose a pattern that becomes a rule, it is the task of the theorist to make this clear." (Carl von Clausewitz, "On War", 1832)

"In the original discovery of a proposition of practical utility, by deduction from general principles and from experimental data, a complex algebraical investigation is often not merely useful, but indispensable; but in expounding such a proposition as a part of practical science, and applying it to practical purposes, simplicity is of the importance: - and […] the more thoroughly a scientific man has studied higher mathematics, the more fully does he become aware of this truth – and […] the better qualified does he become to free the exposition and application of principles from mathematical intricacy." (William J M Rankine, "On the Harmony of Theory and Practice in Mechanics", 1856)

"The more man inquires into the laws which regulate the material universe, the more he is convinced that all its varied forms arise from the action of a few simple principles." (Charles Babbage, "Passages From the Life of a Philosopher", 1864)

"As in the experimental sciences, truth cannot be distinguished from error as long as firm principles have not been established through the rigorous observation of facts." (Louis Pasteur, "Étude sur la maladie des vers à soie", 1870)

"It is of the nature of true science to take nothing on trust or on authority. Every fact must be established by accurate observation, experiment, or calculation. Every law and principle must rest on inductive argument." (Sir John W Dawson, "The Chain of Life in Geological Time", 1880)

"A modern mathematical proof is not very different from a modern machine, or a modern test setup: the simple fundamental principles are hidden and almost invisible under a mass of technical details." (Hermann Weyl, "Unterrichtsblätter für Mathematik und Naturwissenschaften", 1932)

"The fundamental gospel of statistics is to push back the domain of ignorance, prejudice, rule-of-thumb, arbitrary or premature decisions, tradition, and dogmatism and to increase the domain in which decisions are made and principles are formulated on the basis of analyzed quantitative facts." (Robert W Burgess, "The Whole Duty of the Statistical Forecaster", Journal of the American Statistical Association 32 (200), 1937)

"It is always more easy to discover and proclaim general principles than it is to apply them." (Winston Churchill, "The Second World War: The gathering storm", 1948)

"The method of guessing the equation seems to be a pretty effective way of guessing new laws. This shows again that mathematics is a deep way of expressing nature, and any attempt to express nature in philosophical principles, or in seat-of-the-pants mechanical feelings, is not an efficient way." (Richard Feynman, "The Character of Physical Law", 1965)

"No theory ever agrees with all the facts in its domain, yet it is not always the theory that is to blame. Facts are constituted by older ideologies, and a clash between facts and theories may be proof of progress. It is also a first step in our attempt to find the principles implicit in familiar observational notions." (Paul K Feyerabend, "Against Method: Outline of an Anarchistic Theory of Knowledge", 1975)

"Real progress in understanding nature is rarely incremental. All important advances are sudden intuitions, new principles, new ways of seeing." (Marilyn Ferguson, "The Aquarian Conspiracy: Personal and Social Transformation in the 1980s", 1980)

"The word theory, as used in the natural sciences, doesn’t mean an idea tentatively held for purposes of argument - that we call a hypothesis. Rather, a theory is a set of logically consistent abstract principles that explain a body of concrete facts. It is the logical connections among the principles and the facts that characterize a theory as truth. No one element of a theory [...] can be changed without creating a logical contradiction that invalidates the entire system. Thus, although it may not be possible to substantiate directly a particular principle in the theory, the principle is validated by the consistency of the entire logical structure." (Alan Cromer, "Uncommon Sense: The Heretical Nature of Science", 1993)

"Engineering is the application of scientific principles toward practical ends. If the engineering isn't practical, it's bad engineering." (Steve McConnell, "After the Gold Rush: Creating a True Profession of Software Engineering", 1999)

"A small error in the beginning (or in principles) leads to a big error in the end (or in conclusions)." (ancient axiom)

🔭Data Science: Simplicity (Just the Quotes)

"We consider it a good principle to explain the phenomena by the simplest hypothesis possible." (Ptolemy)

"Simplicity of structure means organic unity, whether the organism be simple or complex; and hence in all times the emphasis which critics have laid upon Simplicity, though they have not unfrequently confounded it with narrowness of range." (George H Lewes, "The Principles of Success in Literature", 1865)

"The first obligation of Simplicity is that of using the simplest means to secure the fullest effect. But although the mind instinctlvely rejects all needless complexity, we shall greatly err if we fail to recognise the fact, that what the mind recoils from is not the complexity, but the needlessness." (George H Lewes, "The Principles of Success in Literature", 1865)

"Simplicity and precision ought to be the characteristics of a scientific nomenclature: words should signify things, or the analogies of things, and not opinions." (Sir Humphry Davy, Elements of Chemical Philosophy", 1812)

"The aim of science is always to reduce complexity to simplicity." (William James, "The Principles of Psychology", 1890)

"Let us notice first of all, that every generalization implies in some measure the belief in the unity and simplicity of nature." (Jules H Poincaré, "Science and Hypothesis", 1905)

"Simplicity is the soul of efficiency." (Austin Freeman, "The Eye of Osiris", 1911)

"A theory is the more impressive the greater the simplicity of its premises is, the more different kinds of things it relates, and the more extended its area of applicability." (Albert Einstein, "Autobiographical Notes", 1949)

"As shorthand, when the phenomena are suitably simple, words such as equilibrium and stability are of great value and convenience. Nevertheless, it should be always borne in mind that they are mere shorthand, and that the phenomena will not always have the simplicity that these words presuppose." (W Ross Ashby, "An Introduction to Cybernetics", 1956)

"Scientists whose work has no clear, practical implications would want to make their decisions considering such things as: the relative worth of (1) more observations, (2) greater scope of his conceptual model, (3) simplicity, (4) precision of language, (5) accuracy of the probability assignment." (C West Churchman, "Costs, Utilities, and Values", 1956)

"The central task of a natural science is to make the wonderful commonplace: to show that complexity, correctly viewed, is only a mask for simplicity; to find pattern hidden in apparent chaos." (Herbert A Simon, "The Sciences of the Artificial", 1969)

"For if as scientists we seek simplicity, then obviously we try the simplest surviving theory first, and retreat from it only when it proves false. Not this course, but any other, requires explanation. If you want to go somewhere quickly, and several alternate routes are equally likely to be open, no one asks why you take the shortest. The simplest theory is to be chosen not because it is the most likely to be true but because it is scientifically the most rewarding among equally likely alternatives. We aim at simplicity and hope for truth." (Nelson Goodman, "Problems and Projects", 1972)

"Simplicity does not precede complexity, but follows it." (Alan Perlis, "Epigrams on Programming", 1982)

"Organized simplicity occurs where a small number of significant factors and a large number of insignificant factors appear initially to be complex, but on investigation display hidden simplicity." (Robert L Flood & Ewart R Carson, "Dealing with Complexity: An introduction to the theory and application of systems", 1988)

"The amount of understanding produced by a theory is determined by how well it meets the criteria of adequacy - testability, fruitfulness, scope, simplicity, conservatism - because these criteria indicate the extent to which a theory systematizes and unifies our knowledge." (Theodore Schick Jr.,  "How to Think about Weird Things: Critical Thinking for a New Age", 1995)

"[…] the simplest hypothesis proposed as an explanation of phenomena is more likely to be the true one than is any other available hypothesis, that its predictions are more likely to be true than those of any other available hypothesis, and that it is an ultimate a priori epistemic principle that simplicity is evidence for truth." (Richard Swinburne, "Simplicity as Evidence for Truth", 1997)

"Simplicity isn’t just about reduction. It can also be about augmentation. It consists of removing what isn’t relevant from our models but also of bringing in those elements that are essential to making those models truer." (John Maeda, "The Laws of Simplicity", 2006)

More quotes on "Simplicity" at the-web-of-knowledge.blogspot.com. 

📦Data Migrations (DM): Approaches to Planning a Data Migration for an ERP Implementation

Data Migrations Series

Introduction

ERP implementations are one of the most complex projects to plan as they often imply changes/transformations at different levels (e.g. strategic, processes, data, cultural, technological), span over one or more years, involve many resources that need to be efficiently managed, and often come with important costs for the organization.

One way of handling complexity is to ignore the nonessential in planning by focusing on the important activities/phases, following to go deeper as the project progresses. Another way to handle complexity is to split it at manageable parts – identifying and grouping together components. For example, Data Migration (DM) and Data Quality (DQ) are managed as subprojects, with their own planning. The two strategies can be combined to increase the effect.

Planning a DM cannot be done without looking at the timelines of the ERP implementation and considering the various interfaces to the DQ, however in this post I will focus only on the first two.

The Context

In the context of an ERP implementation there are three main approaches to the planning of a DM – pushing the activities toward the end of the implementation, pushing most activities toward the beginning of the implementation, or splitting the various activities over the whole timeline of the ERP implementation. Borrowing a term from statistics, we can talk about a left-skewed plan, a right-skewed plan, respectively a uniform-distributed plan.

For exemplification I will use a set of Lego pieces grouped together in 3 rows and representing the main phases of an ERP implementation, DM, respectively DQ:

The Lego pieces are a good tool for representing the phases, even they can be mischievous because there’s often no clear delimitation between certain phases as they overlap or repeat over several iterations, and bricks’ length doesn’t necessarily represent the actual duration of the phases. In addition, the phases are oversimplified in order not to clutter the diagrams. The detailed phases will be considered in further posts. The color changes gradually as the activities get closer toward the end.

Left-Skewed Planning

One way to plan a DM is backwards from the Go-Live, the DM activities flowing continuously backwards (the DM bricks are arranged from the end over the implementation bricks) and thus accumulating toward the end of the ERP implementation. This approach is natural considering that the requirements stabilize in the second half of the implementation, the code freeze occurring toward the end. Stabilizing means that the most important code changes were performed, and only minor changes need to be performed, typically bug fixing, refactoring or last-minute changes.

The DM starts with a set of requirements in what concerns the business processes, the data and configuration. Each change to these requirements equate with overwork that need to be performed. Typically, this happens only at entity level, however there can be changes that have impact for the whole or important parts of the migration. Additionally, after each set of changes another dry-run needs to be performed in order test the changes. Therefore, to minimize the volume of rework a DM needs a stable environment – in other words a stabile data model, configuration and requirements.

Thus, the conceptualizing of the migration including the prototyping can start in the first half of the implementation or, for long-running projects, even in the second half of the implementation. Performing the DM without interruptions assures an optimal use and planning of the resources – the resources are continuously working on the project, they are focused toward the end.

On the other side, the accumulation of the activities toward the end can easily lead to problems in what concerns resources’ availability. This type of approach needs a good planning, otherwise the project runs into the risk of having the Go-Live delayed until the stability of the DM is assured, or of going Live with data that don’t have the expected quality. These risks can be alleviated by adding a puffer to DM’s timeline, or by considering one of the other two approaches.

This approach minimizes the various types of waste associated with software projects, and thus the costs associated with waste.

Right-Skewed Planning

To release the pressure existing toward the end of the project, some of the DM activities can be performed toward the beginning of the project – the conceptualization and prototyping, as well the data mapping. One can in theory build also an important part of the DM for the standard functionality, following to address the changes in data model, processes and configuration in subsequent iterations. This could involve a higher volume of rework and more dry-runs, however this depends on the complexity and number of the changes. If a small number of customizations are expected, then this approach may be the best approach. Even in the case of many customization this approach might be something to consider, however the DM costs increase with the number of customization made, and in certain contexts the increase can be exponential.

This approach pushes some of the costs toward the beginning of the implementation, and this can have positive as well negative aspects. For example, it is well-known that ERP implementations involve cost overruns. With this approach the DM costs are assured toward the beginning and one can better get a hold of the budget, at least in theory. As negative aspect could be considered the cases in which an ERP implementation is stopped toward the middle of the project, the incurred costs being thus higher. In the end the main cost-driver are the volume of customizations.

Breaking a DM in two can have several other negative aspects. The data cleaning needs to be broken eventually as well, most probably in the second phase more data enrichment activities need to be considered.

The resources that worked on the first phase might not be available for the second phase. An adequate knowledge transfer might be hard to make, so the second team might need good documentation or time to understand the solution. This can lead to other type of behavior, e.g. rewriting unnecessarily the code, the push for a redesign, and so on.

As the environment stabilizes much later, there is the risk that an important part of the migration need to be reworked/redesigned. In extreme cases might be needed to start from zero. The chances for this to happen are small, though such a case can occur. Probably some of the code, transformation can be reused, though this depends from case to case. Without knowing implementation’s details it’s difficult to estimate the chances for something like this to happen. Sometimes is enough to invalidate a premise considered in design phase. Usually the interplay between several new requirements lead to redesign.

Uniform-distributed Plan

To alleviate the risks from the first two approaches, some of the activities could be uniformly distributed over the whole duration of the ERP implementation. This approach works well when same resources from the vendor side are involved in activities from all the three layers, the nature of the tasks allowing them to work continuously in the project. For example, the consultants working on the DM concept are helping on the mapping of the attributes, as well on data cleansing. When the work on multiple activities isn’t possible then the vendor(s) more likely will have problems in assigning resources to the project. Either the same resources will be assigned for big parts from the projects, incurring thus higher costs, or the resources will be replaced by others, additional learning being involved. In either case the costs are higher.

One of the main dangers of this approach is that certain activities will expand taking the time available, incurring thus higher costs. When the Implementation time is much higher than DM’s duration, the distance between DM’s phases can increase dramatically, being almost impossible to manage resources adequately. Keeping the metaphor of the Lego pieces, it will be thus also more difficult to build a structure on which an edifice can be built. With proper planning and adequate use of resources and knowledge the empty spaces can be incorporated in the structure for project’s advantage.

Even if this approach attempts to even the DM effort over the whole duration of the ERP implementation, performing the activities too early, before requirements stabilized can have an adverse effect.

Personal Approach

Looking back at the projects I worked on, I think I used a hybrid between the 3 approaches. The DM was planed backwards from the Go Live, however the first draft of the DM concept and prototyping was performed at the beginning of the implementation. This assured that the technical solution was working. Being involved in the creation of the data mappings as well in data cleansing, the jumps between activities allowed me to smoothly switch between the various activities, however toward the end of the project this became a bottleneck, the activities being harder to synchronize, and the volume of work could be addressed at that time only with overtime.

With a few exceptions I worked mainly alone on the technical activities, being responsible for the data mappings, design, prototyping, implementation, testing, protocolling, and execution of the DM. I think that more resources would have removed some of the burden but made the planning more complicated and the synchronization even harder. Probably a team of 2-3 people that could cover these activities would provide the optimum balance between costs, effort and quality.

Conclusion

I suppose there is no best solution that will work for all. The three approaches are more an attempt to highlight some of the extreme usages of planning. In an ERP implementation there are so many factors, so many chances for a decision to be an opportunity or a threat. My advice – ponder the various aspects/constraints, choose an approach, and adjust it as the project advances.

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💫ERP Systems: Dynamics AX 2009 (Part I: Deleting Obsolete Companies)

Introduction   

    During implementations, migrations and other projects are created in Dynamics AX temporary companies (aka legal entities, data areas) that aren’t needed anymore once they fulfilled their purpose. Excepting the fact that obsolete companies occupy space in the data center, under certain circumstances they can lead to performance problems. The logical thing to do would be to delete the obsolete companies as long there’s no further demand from the business.    

   In what follows we will look at several methods for deleting obsolete companies. The scripts were tested in Dynamics AX 2009, and more likely they’ll work in coming versions as long the data model behind was kept.

Warning:
    Please note that the scripts are provided “AS IS” only to exemplify a technique and they come without any warranty! Before attempting any of the methods described here, review the comments from “Further Considerations” section!

Method 1: Using DynamcsAX Built-In Functionality   

   Dynamics AX 2009 provides built-in functionality for deleting a company, however when the volume of data in the system goes above a certain limit the functionality starts to perform poorly, even when run directly on an AOS. (It is recommended to run long-running administration jobs directly on the AOS rather than clients.)    For example, it was attempted to use this method to delete several companies in Dynamics AX Test environment. By the first company the deletion job needed a few hours, while by the second company the job hasn’t finished after two days, being thus forced to stop it. After two further failed attempts it came the time to look for another solution.

Warning:
     It seems that this solution can lead to orphaned data (see [1]). So, even if you are using this method, you might need to consider one of the following methods as well.

Method 2: Using sp_MSforEachTable   

  In almost all tables in AX the company is stored in a DataAreaId attribute. Over this attribute the records belonging to a company are logically partitioned. This allows writing a script via the undocumented sp_MSforEachTable stored procedure:

--delete the data for one data area
sp_MSforEachTable @command1 = 'DELETE FROM ? WHERE DataAreaId = ''m01'''

An error with be thrown for the tables that don’t contain the DataAreaId attribute:
Msg 207, Level 16, State 1, Line 1

Invalid column name 'DataAreaId'.The script can be extended to delete in the same step two or more companies:

--delete the data for multiple data areas
 sp_MSforEachTable @command1 = 'DELETE FROM ? WHERE DataAreaId IN (''m01'', ''m02'')'

     During the first test the script needed half of hour to run, however a few tables  in which the company is stored in other attributes remained untouched. One can either search for such tables manually, via a script, or run the built-in AX functionality. We opted for running the built-in functionality, which managed to delete the remaining data relatively fast.

Warning:
Microsoft doesn’t support this method and can be used when the volume of obsolete data is relatively small!    What does it mean relatively small? The most important limitation of this method is the transaction log, considering that the deleted data are logged. One can either change log’s size to accommodate the volume of data to be deleted or run the deletion only for a subset of the tables. (Changing the recovery model to “simple” or “bulk-logged” won’t make a difference.)

   The second important limitation is the available memory, once the available memory is reached SQL Server having to paginate the data, fact that could lead to further disk space consumed.    Other limitations have more with the performance to do, e.g. each deletion is reflected also in the indexes. One might consider for example dropping the indexes before deletion and recreating them afterwards.

Method 3: Using a Cursor    

  Instead of using the undocumented sp_MSforEachTable stored procedure, the loop can be performed via a cursor (see [1]). This method is advantageous when the deletion needs to be performed only for a subset of tables one could use a cursor. The deletion can be grouped together with other activities and run together.

Method 4: Using „Shadow“ Tables    

   When the volume of data available is huge, and the volume of data that remain in the table is small compared with the overall data, it might be useful to consider using “shadow” tables. One can take advantage of the fact that a truncate command performs incomparable better than a delete command.  To use a truncate on a table, the records that need to be kept could be saved temporarily to a copy (aka “shadow”) of the table, the truncate then applied, and the copied records could be moved back. The following scripts exemplify the logic needed to delete the records from InventDim (inventory dimensions) table:

-- (optional) prove the number of records
SELECT count(*) 
FROM dbo.InventDim 
WHERE DataAreaId = 'm01'

-- create the “shadow” table
CREATE TABLE [dbo].[INVENTDIM_Dump](
[INVENTDIMID] [nvarchar](30) NOT NULL,
[INVENTBATCHID] [nvarchar](21) NOT NULL,
[WMSLOCATIONID] [nvarchar](12) NOT NULL,
[INVENTSERIALID] [nvarchar](21) NOT NULL,
[INVENTLOCATIONID] [nvarchar](10) NOT NULL,
[CONFIGID] [nvarchar](10) NOT NULL,
[INVENTSIZEID] [nvarchar](10) NOT NULL,
[INVENTCOLORID] [nvarchar](10) NOT NULL,
[INVENTSITEID] [nvarchar](10) NOT NULL,
[DATAAREAID] [nvarchar](4) NOT NULL,
[RECVERSION] [int] NOT NULL,
[RECID] [bigint] NOT NULL,
[WMSPALLETID] [nvarchar](18) NOT NULL,
[INVENTSTYLEID] [nvarchar](10) NOT NULL
) ON [PRIMARY]

-- copy the data into the “shadow” table
INSERT INTO [dbo].[InventDim_Dump] WITH (TABLOCK)
SELECT *
FROM [dbo].[InventDim] 
WHERE DataAreaId = 'm01'

-- truncate the data frome the main table 
--TRUNCATE TABLE [dbo].[InventDim]

-- copy the data back
INSERT INTO [dbo].[InventDim] WITH (TABLOCK)
SELECT *
FROM [dbo].[InventDim_Dump]

-- (optional) prove whether the IDs were correctly copied 
SELECT count(*)
FROM [dbo].[InventDim] A
JOIN [dbo].[InventDim_Dump] B
ON A.recid = B.RECID 
AND A. DATAAREAID = B.DATAAREAID 
WHERE A.DataAreaId = 'm01'

-- drop the „shadow“ table 
--DROP TABLE[dbo].[InventDim_Dump]

  

   As can be seen the “shadow” tables are simplified versions of the original tables, without constraints or indexes. They can be eventually created in another schema or even other database.   

   Except the script for table’s creation in the other scripts table’s name can be easily replaced in the editor via the search and replace functionality, trick that reduces considerably the time needed for development. I needed on average 5 minutes for each table, plus 3-4 hours for further tests.    

   The optional steps are more for exemplification and can be eventually removed.  

   The Tablock hint used in inserts provides better performance and minimizes the volume of data logged.    

   I used this method only for the tables having more than 3 million records, around 50 tables in total. Between them there were a few tables having 20-200 GB worth of records. I started with these big tables and figured out that also smaller tables could benefit from this method. A few minutes gained for each small table resulted in the end in a gain of a couple of hours.

   The remained records were 0-25% of the initial tables.   

   In theory, these steps could be performed within a cursor in which the creation of the “shadow” tables could be automated via table metadata as well. This approach will pay-off especially when the schema is not fixed, or the procedure needs to be repeated on different schemas.

Method 5: Delete Records in Batches    

   There will be a point beyond which the performance provided by the fourth method will deprecate considerably. This point is based on the volume of records available in the table, and the records needed to be inserted back and forth. Without further tests, I suppose that this point lies in the 50-75% interval. Beyond this point for big tables in range of 10x or 100x GB it might be useful to delete the data in batches. A push in this direction might be constrained by the need to shrink the transaction log in between the deletes. The query could be written as follow:

-- deleting top x records 
DELETE top 10000
FROM dbo.InventDim WITH (TABLOCK)
WHERE DataAreaId = ‘m01’

   The query can be included in a loop or run manually until no records are returned. It can be tested with different batch sizes to determine the best solution. In between is recommended to check also the growth of the log file and truncate it accordingly when needed.

Method 6: Using X++ Code  

    For those having some basic knowledge of X++ and Dynamics AX classes, a solution based on deleting data via AX code could prove to be a better solution as standard functionality can be leveraged, functionality that eventually considers also the business logic implemented. The downside is the code that need to be written for this purpose, however there are already some examples available on the web (see [4]).

Hint:
In AX 2012 built-in support for batch deletes was added via the delete_from statement (see [3]).

Further Considerations    

   Before attempting a deletion, it might be useful to analyze how many records will be deleted from each table, and eventually devise different scenarios for specific table categories. To get the number of records one can use either the built-in functionality from AX or use the sp_MSforEachTable stored procedure and export the results to text, following to overwork the data further in Excel:

-- listing the number of records per company 
sp_MSforEachTable @command1 = 'SELECT dataareaid, ''?'' table_name, count(*) no_records FROM ? WHERE DataAreaId IN (''m01'', ''m02'') GROUP BY dataareaid'

The results can be used also to approximate the space occupied by the data.   

   Independently of the method used it is recommended to restrict users‘ access to the system and to deactivate the scheduled AX or SQL Server jobs. This will ensure that no blockings will occur in the system during the respective time.    

   As data are synchronized between the AOS’s and the database, it is recommended to shut down the not needed AOS services before the deletions are performed, and restart them once all activities were performed.   

   To minimize the risks associated with the loss of data it’s recommended to perform a backup of the database(s) before performing any changes.    

   By deleting the data directly on the database, the business logic from AX (including customizations) is skipped. In theory this can lead to logical inconsistencies, however considering that all the data for a company are deleted, the risks are very small, unless intercompanies are involved.   

   After the data are deleted it is recommended to recreate the indexes and update the statistics on the tables.  

   Check whether the transaction log can accommodate the volume of records to be deleted! In extreme cases your SQL Server might crash! From this consideration it might be advantageous to delete only a company at a time.    

   Based on the volume of data available in the transaction log it might be needed to truncate the log(s) between the steps, as well at the end.  

   After the principle “better safe than sorrow”, it might be a good idea to check the physical and logical consistency of the data before letting the users in.   

  To minimize the impact on the business, it is recommended to perform the deletion outside the working hours, otherwise the action can lead to blocking and even deadlocks in the system.     Always attempt to use standard functionality and resort to other methods only when there’s no way around it.

  It is recommended to always test the scripts thoroughly in the test environment before attempting their productive usage!

References:
[1] Microsoft Dynamics AX Technical Support Blog (2010) How to delete orphaned data remained from deleted company?, by Martin Falta [Online] Available from: https://blogs.msdn.microsoft.com/emeadaxsupport/2010/12/09/how-to-delete-orphaned-data-remained-from-deleted-company/
[2] Art of Creation (2010) Delete an AX company on SQL [Online] Available from: http://www.artofcreation.be/2010/02/03/delete-an-ax-company-on-sql/
[3] MSDN (2012) delete_from Statement [Online] Available from: https://msdn.microsoft.com/en-us/library/aa624886.aspx[
4] Kevin’s blog (2017) Dynamics Ax 2012 History cleanup, by Kevin Roos [Online] Available from: https://www.kevinroos.be/2017/07/dynamics-ax-2012-history-cleanup/

📦Data Migrations (DM): Guiding Principles

Data Migrations Series

Introduction

“An army of principles can penetrate where an army of soldiers cannot."
Thomas Paine

In life as well in IT principles serve as patterns of advice in form of general or fundamental ideas, truths or values stated in a context-independent manner. They can be used as guidelines in understanding and modeling the reality, the world we live in. With the invasion of technologies in our lives principles serve as a solid ground on which we can build castles – solutions for our problems. Each technology comes with its own set of principles that defines in general terms its usage. That's why most of the IT books attempt to catch these sets of principles. Unfortunately, few of the technical writers manage to define some meaningful principles and showcase their usages.

Many of the ideas considered as principles in papers on Data Migration (DM) are at best just practices, and some can be considered as best/good practices. Just because something worked good in a previous migration doesn’t mean automatically that the idea behind the respective decision turns automatically in a principle. Some of the advices advanced are just lessons learned in disguise. Principles through their generality apply to a broad range of cases, while practices are more activity specific.

A DM through its nature finds its characteristics at the intersection of several area - database-based architecture design, ETL workflows, data management, project management (PM) and services. From these areas one can pull a set of principles that can be used in building DM architectures.

Architecture Principles

“Architecture starts when you carefully put two bricks together.”
Ludwig Mies van der Rohe

There are several general principles that apply to the architecture of applications, independently of the technologies used or the industry, e.g. research first, keep it simple/small, start with the end in mind, model first, design to handle failure, secure by design (aka safety first), prototype, progress iteratively, focus on value, reuse (aka don't reinvent the wheel), test early, early feedback, refactor, govern, validate, document, right tool – right people, make it to last, make it sustainable, partition around limits, scale out, defensive coding, minimal intervention, use common sense, process orientation, follow the data, abstract, anticipate obsolescence, benchmark, single-responsibility, single dispatch, separation of concerns, right perspective.

To them add a range of application design characteristics that can be considered as principles as well: extensibility, modularity, adaptability, reusability, repeatability, modularity, performance, revocability, auditability, subject-orientation, traceability, robustness, locality, heterogeneity, consistency, atomicity, increased cohesion, reduced coupling, monitoring, usability, etc. There are several principles that can be transported from problem solving into design - divide and conquer, prioritize, system’s approach, take inventory, and so on.

A DM’s architecture has more to do with a data warehouse as it relies heavily on ETL tasks and data need to be stored for various purposes. Besides the principles of good database design, a few other principles apply: model (the domain) first, denormalize, design for performance, maintainability and security, validate continuously. From ETL area following principles can be considered: single point of processing, each step must have a purpose, minimize touch points, rest data for checkpoints, leverage existing knowledge, automate the steps, batch processing.

 In addition, considering their data-specific character, a DM can be regarded as one or several data products, though in contrast with typical data products DM have typically a limited purpose. From this area following principles could be considered: build trust with transparency, blend in, visualize the complex.

Data Management Principles

Considering that a DM’s focus is an organization's data, some principles need to focus on the management and governance of Data. Data Governance together with Data Quality, Data Architecture, Metadata Management, Master Data Management are functions of Data Management. The focus is on data, metadata and their lifecycle, on processes, ownership and roles and their responsibilities. With this in mind there can be defined several principles supposed to facilitate the functions of Data Management: manage data as asset, manage data lifecycle, the business owns the data, integration across the organization, make data/metadata accessible, transparent and auditable processes, one source of truth.

As part of DM there are customer, employee and vendor information which fall under the General Data Protection Regulation (GDPR) EU 2016/679 regulation which defines the legal framework for data protection and privacy for all individuals within the European Union (EU) and the European Economic Area (EEA) as well the export of personal data outside the EU and EEA. The regulation defines a set of principles that make its backbone: fairness, lawfulness and transparency, purpose limitation, data minimization, accuracy, storage limitation, integrity and confidentiality, accountability [6].

Overseas, the US Federal Trade Commission (FTC) issued in 2012, a report recommending organizations design and implement their own privacy programs based on a set of best practices. The report reaffirms the FTC’s focus on Fair Information Processing Principles, which include notice/awareness, choice/consent, access/participation, integrity/security, enforcement/redress [6].

Project Management (PM) Principles

"Management is doing things right […]"
Peter Drucker

A DM though its characteristics is a project and, to increase the chances of success, it needs to be managed as a project. Managing DM as a project is one of the most important principles to consider. The usage of a PM framework will further increase the chances of success, as long the framework is adequate for the purpose and the organization team is able to use the framework. PMI, Prince2, Agile/Scrum/Kanban are probably the most used PM methodologies and they come with their own sets of principles.

In general, all or some of the PM principles apply independently on whether is used alone or in combination with other PM methodologies: a single project manager, an informed and supportive management, a dedicated team of qualified people to do the work of the project, clearly defined goals addressing stakeholders’ priorities, an integrated plan and schedule, as well a budget of costs and/or resources required [1].

On the other side, an agile approach could prove to be a better match for a DM given that requirements change a lot, frequent and continuous deliveries are needed, collaboration is necessary, agile processes as well self-organizing teams can facilitate the migration. These are just a few of the catchwords that make the backbone of the Agile Manifesto (see [3]).

An agile form of Prince2 could be something to consider as well, especially when Prince2 is used as methodology for other projects. For Prince2 are the following principles to consider: continued business justification, learn from experience, defined roles and responsibilities, manage by stages, management by exception, focus on products, tailor to suit the project environment [2].

All these PM principles reveal important aspects to ponder upon, and maybe with a few exceptions, all can be incorporated in the way the DM project is managed.

Service Principles

Considering the dependencies existing between the DM and Data Quality as well to the broader project, a DM can have the characteristics of a service. It’s not an IT Service per se, as IT only supports technically and eventually from a PM perspective the project. Even if a DM is not a ITSM service, some of the ITIL principles can still apply: focus on value, design for experience, start where you are, work holistically, progress iteratively, observe directly, be transparent, collaborate and keep it simple [4].

Conclusion

“Obey the principles without being bound by them.”
Bruce Lee

Within a DM all the above principles can be considered, though the network of implication they create can easily shift the focus from the solution to the philosophical aspects, and that’s a marshy road to follow. Even if all principles are noble, not all can be considered. It would be utopic to consider each possible principle. The trick is to identify the most “important” principles (principles that make sense) and prioritize them according to existing requirements. In theory, this is a one-time process that involves establishing a “framework” of best/good practices for the DM, in next migrations needing only to consider the new facts and aspects.

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References:
[1] “Principles of project management”, by J. A. Bing, PM Network, 1994 (link)
[2] Axelos (2018) What is PRINCE2? (link)
[3] Agile Manifesto (2001) Principles behind the Agile Manifesto (link)
[4] Axelos (2018) ITIL® Practitioner 9 Guiding Principles (link)
[5] The Data Governance Institute (2018) Goals and Principles for Data Governance (link) 
[6] Navigating the Labyrinth: An Executive Guide to Data Management, by Laura Sebastian-Coleman for DAMA International, Technics Publications, 2018 (link)  

🔬Data Science: Data Model (Definitions)

"simplified and approximative description of a system or process, based on a finite set of essential variables and their analytically definable behavior." (Teuvo Kohonen, "Self-Organizing Maps" 3rd Ed., 2001)

"(i) An abstract, self-contained logical definition of the data structures and associated operators that make up the abstract machine with which users interact (such as the relational model of data). (ii) A model of the persistent data of some enterprise" (Keith Gordon, "Principles of Data Management", 2007)

"A means of encapsulating the data elements that were decided on by the business experts, in conjunction with the data stewards and IT professionals. The data models reflect an organization’s business as represented in the data." (Tony Fisher, "The Data Asset", 2009)

"An abstraction of how individual data elements relate to each other. It visually depicts how the data is to be organized and stored in a database. A data model provides the mechanism to document and understand how data is organized." (Laura Reeves, "A Manager's Guide to Data Warehousing", 2009)

"An organization of data that describes the relationships among primitive and composite data elements." (Toby J. Teorey, "Database Modeling and Design" 4th Ed., 2010)

"A model of the structure (and to some extent the content) of a database and at least some of the rules governing the data therein." (Graham Witt, "Writing Effective Business Rules", 2012)

"A data model is a visual representation of data content and the relationships, created for purposes of understanding how data is or might be organized, and for ensuring the comprehensibility and usability of that way of organizing data." (Laura Sebastian-Coleman, "Measuring Data Quality for Ongoing Improvement", 2013)

"Represents data objects and their relationships with each other. Data models form the basis for data integration at the conceptual level as well as the improvement of data quality, such as with regard to the reduction of data redundancy. Data models are one component of the data architecture." (Boris Otto & Hubert Österle, "Corporate Data Quality", 2015)

"A template formalizing the relationship between an input and an output. Its structure is fixed but it also has parameters that are modifiable; the parameters are adjusted so that the same model with different parameters can be trained on different data to implement different relationships in different tasks." (Ethem Alpaydın, "Machine learning : the new AI", 2016)

"A visual means of depicting data and its relationship to other data." (Gregory Lampshire et al, "The Data and Analytics Playbook", 2016)

"An abstract representation of a subject that looks and/or behaves like all or part of the original." (George Tillmann, "Usage-Driven Database Design: From Logical Data Modeling through Physical Schmea Definition", 2017)

"In the context of machine learning, a model is a representation of a pattern extracted using machine learning from a data set. Consequently, models are trained, fitted to a data set, or created by running a machine learning algorithm on a data set. Popular model representations include decision trees and neural networks. A prediction model defines a mapping (or function) from a set of input attributes to a value for a target attribute. Once a model has been created, it can then be applied to new instances from the domain. For example, in order to train a spam filter model, we would apply a machine learning algorithm to a data set of historic emails that have been labeled as spam or not spam. Once the model has been trained it can be used to label (or filter) new emails that were not in the original data set." (John D Kelleher & Brendan Tierney, "Data science", 2018)

"An abstract model that describes how data is presented and used." (Piethein Strengholt, "Data Management at Scale", 2020)

"A logical map that represents the inherent properties of the data independent of software, hardware or machine performance considerations. The model shows data elements grouped into records, as well as the association around those records." (Information Management)

"Defines how data is structured, related, and standardized for the purpose of extracting meaningful insight." (Insight Software)

"A data model is an abstract model that organizes elements of data and standardizes how they relate to one another. Their properties are generally governed by properties of the real world entities." (kloudless)

🔬Data Science: Multilayer Perceptron (Definitions)

"A neural net composed of three or more slabs (and therefore two or more layers of weighted connection paths); such nets are capable of solving more difficult problems than are single layer nets. They are often trained by backpropagation." (Laurene V Fausett, "Fundamentals of Neural Networks: Architectures, Algorithms, and Applications", 1994)

"A fully connected feedforward NN with at least one hidden layer that is trained using back-propagation algorithmic techniques." (Ioannis Papaioannou et al, "A Survey on Neural Networks in Automated Negotiations", Encyclopedia of Artificial Intelligence, 2009)

"A kind of feed-forward neural network which has at least one hidden layer of neurons." (Fernando Mateo et al, "A 2D Positioning Application in PET Using ANNs", Encyclopedia of Artificial Intelligence, 2009)

"A neural network that has one or more hidden layers, each of which has a linear combination function and executes a nonlinear activation function on the input to that layer." (Robert Nisbet et al, "Handbook of statistical analysis and data mining applications", 2009)

"It has a layered architecture consisting of input, hidden and output layers. Each layer consists of a number of perceptrons." (Siddhartha Bhattacharjee et al, "Quantum Backpropagation Neural Network Approach for Modeling of Phenol Adsorption from Aqueous Solution by Orange Peel Ash", 2013)

"A type of neural network. The MLP is the most common, and arguably the simplest, neural network used for classification." (Meta S Brown, "Data Mining For Dummies", 2014)

"An artificial neural network model with feed forward architecture that maps sets of input data onto a set of desired outputs iteratively, through the process of learning. A MLP consists of an input layer of neurons, one or more hidden layers of neurons and an output layer of neurons, where each layer is fully connected to the next layer." (Eitan Gross, "Stochastic Neural Network Classifiers", 2015) 

"an important class of ANN that typically consists of the input layer, one or more hidden layers of computation nodes, and an output layer. The input signal propagates through the network in a forward direction, on a layer-by-layer basis." (Pablo Escandell-Montero et al,"Artificial Neural Networks in Physical Therapy", 2015)

"Arguably the most popular artificial neural network model. It is usually composed by three or four layers of units. Each unit is fully connected to the units of the previous layer. Learning is customarily performed via the backpropagation rule." (D T Pham & M Castellani, "The Bees Algorithm as a Biologically Inspired Optimisation Method", 2015)

"Is an ANN type that requires a reference to learn patterns. It is trained using (error) back propagation algorithm." (Kandarpa K Sarma, "Learning Aided Digital Image Compression Technique for Medical Application", 2016)

"MLP is a feed forward neural network with one or more layers between input and output layer and are used to solve non-linearly separable problems. MLPs are trained using the back propagation algorithm. MLPs are widely used in pattern classification, recognition, prediction, etc." (Mridusmita Sharma & Kandarpa K Sarma, "Soft-Computational Techniques and Spectro-Temporal Features for Telephonic Speech Recognition: An Overview and Review of Current State of the Art", 2016)

🔬Data Science: Neuron (Definitions)

[Chaotic neuron:] "An artificial neuron whose output is calculated with the use of a chaotic output function." (Nikola K Kasabov, "Foundations of Neural Networks, Fuzzy Systems, and Knowledge Engineering", 1996)

[Oscillatory neuron:] "An artificial neuron built up of two elements (or two groups of elements), one of them being excitatory and the other inhibitory. Its functioning is described as oscillation, characterized by three parameters: frequency; phase; amplitude." (Nikola K Kasabov, "Foundations of Neural Networks, Fuzzy Systems, and Knowledge Engineering", 1996)

"A nerve cell in the physiological nervous system." (Guido J Deboeck and Teuvo Kohonen, "Visual explorations in finance with self-organizing maps", 2000)

[Hidden neuron:] "Usually a nonlinear (or linear) processing element with no direct connections to either inputs or outputs. It often provides the learning capacity of the neural network." (Guido Deboeck & Teuvo Kohonen (Eds), "Visual Explorations in Finance with Self-Organizing Maps", 2000)

"any of the numerous types of specialized cell in the brain or other nervous systems that transmit and process neural signals. The nodes of artificial neural networks are also called neurons." (Teuvo Kohonen, "Self-Organizing Maps 3rd Ed.", 2001)

"A single processing element in a neural network. The most common form of neuron has two basic parts: a summation function that receives inputs and a transfer function that processes inputs and passes the processed values to the next layer of neurons. If the neuron is in the last layer of the network, the output is the final estimate of the dependent variable for that input vector or case." (David Scarborough & Mark J Somers, "Neural Networks in Organizational Research: Applying Pattern Recognition to the Analysis of Organizational Behavior", 2006)

"the elementary processing unit that composes an ANN." (Pablo Escandell-Montero et al, "Artificial Neural Networks in Physical Therapy", 2015)

"A neuron takes a number of input values (or activations) as input and maps these values to a single output activation. This mapping is typically implemented by applying a multi-input linear-regression function to the inputs and then pushing the result of this regression function through a nonlinear activation function, such as the logistic or tanh function." (John D Kelleher & Brendan Tierney, "Data science", 2018)

"Specialized brain cell that integrates inputs from other neurons and sends outputs to other neurons." (Terrence J Sejnowski, "The Deep Learning Revolution", 2018)

"A unit in a neural net whose function (such as y = tanh(w.dot(x))) takes multiple inputs and outputs a single scalar value. This value is usually the weights for that neuron (w or wi) multiplied by all the input signals (x or xi) and summed with a bias weight (w0) before applying an activation function like tanh. A neuron always outputs a scalar value, which is sent to the inputs of any additional hidden or output neurons in the network. If a neuron implements a much more complicated activation function than that, like the enhancements that were made to recurrent neurons to create an LSTM, it is usually called a unit, for example, an LSTM unit." (Hobson Lane et al, "Natural Language Processing in Action: Understanding, analyzing, and generating text with Python", 2019)

"An artificial neuron is a model of a neuron present in an animal brain that is perceived as a mathematical function." (Hari K Kondaveeti et al, "Deep Learning Applications in Agriculture: The Role of Deep Learning in Smart Agriculture", 2021)

🔬Data Science: Pattern Recognition (Definitions)

"The categorization of patterns in some domain into meaningful classes. A pattern usually has the form of a vector of measurement values." (Guido Deboeck & Teuvo Kohonen (Eds), "Visual Explorations in Finance with Self-Organizing Maps 2nd Ed.", 2000)

"in the most general sense the same as artificial perception." (Teuvo Kohonen, "Self-Organizing Maps" 3rd Ed., 2001)

"The operation and design of systems that recognize patterns in data." (Craig F Smith & H Peter Alesso, "Thinking on the Web: Berners-Lee, Gödel and Turing", 2008)

"Research area that enclose the development of methods and automatized techniques for identification and classification of samples in specific groups, in accordance with representative characteristics." (Paulo E Ambrósio, "Artificial Intelligence in Computer-Aided Diagnosis",  Encyclopedia of Artificial Intelligence, 2009)

"The process of identifying patterns in data via algorithms to make predictions within a subject area." (Jason Williamson, Getting a Big Data Job For Dummies, 2015)

"A branch of machine learning that recognizes and separates the patterns of one class from the other." (Mridusmita Sharma & Kandarpa K Sarma, "Soft-Computational Techniques and Spectro-Temporal Features for Telephonic Speech Recognition: An Overview and Review of Current State of the Art", 2016)

"A pattern is a particular configuration of data; for example, ‘A’ is a composition of three strokes. Pattern recognition is the detection of such patterns." (Ethem Alpaydın, "Machine learning : the new AI", 2016)

"Pattern Recognition in the discipline which tries to find the classes in the datasets of the various applications and it is the major building block of artificially intelligent systems." (Vandana M Ladwani, "Support Vector Machines and Applications", 2017)

"identifying patterns in data via algorithms to make predictions of new data coming from the same source." (Analytics Insight)