Saturday, June 6, 2009

Stress testing your model - Part 1/3

So, you've built a model. You have been careful about understanding your data, transforming it appropriately, used the right modeling technique, done an independent validation (if it is an empirical model) and now you are ready to use the model to make forecasts, drive decisions, etc.

Wait. Not so fast. Before the model is ready for prime time, you need to make sure that the model is robust. What defines a robust model?
- the inputs should cover not just the expected events but also extreme events
- the model should not break down (i.e., mispredict) when the inputs turn extreme (Well, no model can be expected to perform superbly when the inputs turn extreme. If models could do that, the events wouldn't be termed extreme events. But the worst thing that a model can do is provide an illusion of normal (english usage) outputs when the inputs are extreme.

I want to share some of the techniques that are used for understanding the robustness of the models, what I like about them and what I don't.

1. When it comes to empirical models, one of the most useful techniques is Out of Sample Validation. This is done by building the model on one data set and validating the algorithm on another. For the truest validation, the validation dataset should be independent of the build, should be drawn from a different time period. "Check-the-box" type validation is when you validate the model on a portion of the build sample itself. Such validation often holds and just as often offers a false sense of security, because in real terms, you have really not validated anything.
Caveat: Out of sample validation is of no use if the future is going to look very different from the past. Validating a model to predict the probability of mortgage default using conventional mortgages data would have been of no use in a world where no-documentation mortgages and other exotic-term mortgages were being marketed.

The other two approaches I want to discuss are Sensitivity Analysis and Monte Carlo Simulation. I will cover them in subsequent posts.

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