Figure 6.6 shows such a tree, grown to include many detailed factors. This has an accuracy on the training set of 83%, better than the smaller tree. But when we apply this algorithm to the test data its accuracy drops to 81%, the same as the small tree, and its Brier score is 0.150, clearly worse than the simple tree’s 0.139. We have adapted the tree to the training data to such a degree that its predictive ability has started to decline.
