Learning with Class Skews and Small Disjuncts

One of the main objectives of a Machine Learning – ML – system is to induce a classifier that minimizes classification errors. Two relevant topics in ML are the understanding of which domain characteristics and inducer limitations might cause an increase in misclassification. In this sense, this work analyzes two important issues that might influence the performance of ML systems: class imbalance and errorprone small disjuncts. Our main objective is to investigate how these two important aspects are related to each other. Aiming at overcoming both problems we analyzed the behavior of two over-sampling methods we have proposed, namely Smote + Tomek links and Smote + ENN. Our results suggest that these methods are effective for dealing with class imbalance and, in some cases, might help in ruling out some undesirable disjuncts. However, in some cases a simpler method, Random over-sampling, provides compatible results requiring less computational resources.