FINDING SMALL EQUIVALENT DECISION TREES IS HARD

Abstract

Two decision trees are called decision equivalent if they represent the same function, i.e., they yield the same result for every possible input. We prove that given a decision tree and a number, to decide if there is a decision equivalent decision tree of size at most that number is NP-complete. As a consequence, finding a decision tree of minimal size that is decision equivalent to a given decision tree is an NP-hard problem. This result differs from the well-known result of NP-hardness of finding a decision tree of minimal size that is consistent with a given training set. Instead our result is a basic result for decision trees, apart from the setting of inductive inference. On the other hand, this result differs from similar results for BDDs and OBDDs: since in decision trees no sharing is allowed, the notion of decision tree size is essentially different from BDD size.