27 Mar 2017

Bruce's Modelling Gripes, No.1: Unclear or confused modelling purpose

OK, maybe I am just becoming a grumpy ol man, but I thought I would start a series about tendencies in my own field that I think are bad :-), so here goes...

Modelling Gripe No.1: Unclear or confused modelling purpose

A model is a tool. Tools are useful for a particular purpose. To justify a new model one has to show it is good for its intended purpose.  Some modelling purposes include: prediction, explanation, analogy, theory exploration, illustration... others are listed by Epstein [1]. Even if a model is good for more than one purpose, it needs to be justified separately for each purpose claimed.

So here are 3 common confusions of purpose:

1.    Understanding Theory or Analogy -> Explanation. Once one has immersed oneself in a model, there is a danger that the world looks like this model to its author. Here the temptation is to immediately jump to an explanation of something in the world. A model can provide a way of looking at some phenomena, but just because one can view some phenomena in a particular way does not make it a good explanation.

2.    Explanation -> Prediction. A model that establishes an explanation traces a (complex) set of causal steps from the model set-up to outcomes that compare well with observed data. It is thus tempting to suggest that one can use this model to predict this observed data. However, establishing that a model is good for prediction requires its testing against unknown data many times – this goes way beyond what is needed to establish a candidate explanation for some phenomena.

3.    Illustration -> Understanding Theory. A neat illustration of an idea, suggests a mechanism. Thus the temptation is to use a model designed as an illustration or playful exploration as being sufficient for the purpose of a Understanding Theory. Understanding Theory involves the extensive testing of code to check the behaviour and any assumptions. An illustration, however suggestive, is not that rigorous. For example, it maybe that an illustrated process only appears under very particular circumstances, or it may be that the outcomes were due to aspects of the model that were thought to be unimportant. The work to rule out these kinds of possibility is what differentiates using a model as an illustration from modelling for Understanding Theory.

Unfortunately many authors are not clear in their papers about specifying exactly for which purpose they are justifying their model.  Maybe they have not thought about it, maybe they are just confused and maybe they are just being sloppy (e.g. assuming because its good for one purpose it is good for another).

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