"A bad model is a combination of assertions, some factual, others conjectural, and others plainly false but convenient. [..] By definition, a bad model does not give power to see-accurately, deeply, or at all-into the actual situa- tion, but only into the assertions embodied in the model. Thus, if the use of a bad model provides insight, it does so not by revealing truth about the world but by revealing its own assumptions and thereby causing its user to go learn something about the world." (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
"A management system driven by a bad model must not be tested by using the model as if it were true. By presumption, the model is a deficient picture of reality, and it presents the management system with the easiest possible test because it, unlike the cruel world, satisfies the system's assumptions. But a bad model can be used as a vehicle for a fortiori arguments in an evaluation of a system of which it is a part [...]" (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
"A scale model is a bad model in the sense used here: It is grossly discrepant with reality, if only because it is far too small for anyone to live in. Nonetheless, it can do a good job of selling the idea-the project-of which it is but an illustration, by conveying aspects of the idea concretely." (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
"Just because a model is bad, however, does not mean it is useless. [...] A bad model can be used to construct correct paths from premises to conclusions, but because its relations to reality are questionable, it can only do so in a few ways-at least, ways that permit useful conclusions with respect to reality." (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
"Often, though, a policy or systems analyst is stuck with a bad model, that is, one that appeals to the analyst as adequately realistic but which is either: 1) contradicted by some data or is grossly implausible in some aspect it purports to represent, or 2) conjectural, that is, neither supported nor contradicted by data, either because data do not exist or because they are equivocal. [...] A model may have component parts that are not bad, but if, taken as a whole, it meets one of these criteria, it is a bad model." (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
"One might hope for a standard of consistency instead of a lack of inconsistency, but, as a practical matter, no one can make every consistency check, so a stingent lack of inconsistency is the most one can ask for. Even this need not be simple." (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
"Some readers have argued that the criticism implied by the term "bad models" is undeserved because they can be used appropriately in some cases. [...] If the logic works, the use is appropriate; if it fails, the use in inappropriate: (Cost effectiveness is a separate issue.) As for the pejorative connotation of the term bad model, perhaps we should admit that many useful models would be embarrassments to scientists, from whom we got the idea of a model, but whose job is to improve the match between models and reality." (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
"Sometimes the proprietors of a bad model claim that parts of it are facts, not just beliefs. Evaluation then amounts to determining if facts support the claims, and disciplines like statistics have tools for this task. The difficulty of using statistical tools will vary depending on the problem." (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
"This stricture - that a bad model can only suggest-is stronger than it may appear. Bad models produce numbers, and thus present an unbearable temptation to use those numbers as if they do more than suggest. They cannot. If a model is bad as defined here, and the specific numbers it produces cannot be buttressed by some other arguments, then the numbers have no meaning except as illustration of the consequences that flow from the model's assumptions." (James S Hodges, "Six (or So) Things You Can Do with a Bad Model", 1991)
