If you are running out of memory or your execution times seem inordinately high, look at how your average program size is changing over time. If programs are growing extremely fast, you may want to implement some form of bloat control (see Section 11.3 ). Naturally, long runs may simply be the result of the population being very large or the fitness evaluation being slow. In these cases, you may find the techniques described in Chapter 10 helpful.
Controlling bloat is also important if your goal is to find a comprehensible model, since in practice smaller models are easier to understand. A large model will not only be difficult to understand but also may over-fit the training data (Gelly, Teytaud, Bredeche, and Schoenauer, 2006).