Loaded words and modeling

Here is a nice review by Burton Malkiel of “Models Behaving Badly” by Emanuel Derman. The models of the title are from the world of finance: how are assets priced?

I am a layperson to the world of finance, so I find it difficult how to apportion “blame” for financial crises on faulty models or fraudulent inputs to them. Certainly history is littered with financial crises, so the influence of modern modeling alone cannot explain everything.

In any case, I just want to take this opportunity for a small lament that the beautiful act of modeling must be dragged through the mud by a financial crisis in this way. It would be fair to say that I am almost fanatical about the virtues of the concept and practice of modeling. I believe that modeling is inescapable. The world is complicated. Our senses deliver so much information, our mental apparatus must work so hard, that to process the world around us is to model. It is too much to ask that we understand everything; we have to understand a version of everything that is not so complex as the world.

This is also why economics works with models. We don’t have a scale replica of the world that we can play with to see how this affects that. We have to build a scale replica from scratch, using our best judgment to push insistently at the boundary between complexity (so that we can understand our model) and usefulness (so that we can make something from it).

In a way we are much luckier in economics than in finance. Progress in economic theory comes as our models are improved upon and refined, but we are more able to iterate forward because our models are not embedded in a Leviathan global finance industry that depends on their continued function. Creative destruction of old models is hard when the house comes down with them.

With all this in mind I want to highlight this passage from the review:

He sums up his key points about how to keep models from going bad by quoting excerpts from his “Financial Modeler’s Manifesto” (written with Paul Wilmott), a paper he published a couple of years ago. Among its admonitions: “I will always look over my shoulder and never forget that the model is not the world”; “I will not be overly impressed with mathematics”; “I will never sacrifice reality for elegance”; “I will not give the people who use my models false comfort about their accuracy”; “I understand that my work may have enormous effects on society and the economy, many beyond my apprehension.”

How many of these will I accept for economics? Certainly the first; the model is not reality. Certainly the second; math is helpful in model-building but is not the point of model-building. The fourth and fifth are hard to argue with.

The third I don’t like. Everything that we do must sacrifice reality. The test of a model is not its realism (a realistic model airplane would be no fun at all). All models are unrealistic because all models are wrong. Of course elegance is not the test of a model either, except that an elegant model is one that illuminates a relationship in a clear way by cutting to the heart of what matters.

Anyway, the point is that I think that “model” is not a dirty word. I feel possessive about “modeling” much the same way as I feel possessive about “rationality” – what they mean to me is important and wonderful and I hate to see them sullied by misrepresentation that stems from their overlap with the real-world ideas of modeling and rationality. I wish that all of the things like these could have their own words that are not borrowed from natural language.

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