6. HEAD-DT: Fitness Function Analysis

In Chap. 4, more specifically in Sect. 4.​4, we saw that the definition of a fitness function for the scenario in which HEAD-DT evolves a decision-tree algorithm from multiple data sets is an interesting and relevant problem. In the experiments presented in Chap. 5, Sect. 5.​2, we employed a simple average over the F-Measure obtained in the data sets that belong to the meta-training set. As previously observed, when evolving an algorithm from multiple data sets, each individual of HEAD-DT has to be executed over each data set in the meta-training set. Hence, instead of obtaining a single value of predictive performance, each individual scores a set of values that have to be eventually combined into a single measure. In this chapter, we analyse in more detail the impact of different strategies to be used as fitness function during the evolutionary cycle of HEAD-DT. We divide the experimental scheme into two distinct scenarios: (i) evolving a decision-tree induction algorithm from multiple balanced data sets; and (ii) evolving a decision-tree induction algorithm from multiple imbalanced data sets. In each of these scenarios, we analyse the difference in performance of well-known performance measures such as accuracy, F-Measure, AUC, recall, and also a lesser-known criterion, namely the relative accuracy improvement. In addition, we analyse different schemes of aggregation, such as simple average, median, and harmonic mean.