Empirical studies have shown there is wide variation in cost (e.g., of devising and executing test cases) and effectiveness (at finding faults) across existing state-based coverage criteria. As these criteria can be considered as executing the control flow structure of the statechart, we are attempting to investigate how data flow information can be used to improve their cost-effectiveness. This article presents a comprehensive methodology to perform data flow analysis of UML statecharts, applies it to the round-trip path (transition tree) coverage criterion and reports on two case studies. The results of the case studies show that data flow information can be used to select the best cost-effective transition tree when more than one satisfies the transition tree criterion. We further propose a more optimal strategy for the transition tree criterion, in terms of cost and effectiveness. The improved tree strategy is evaluated through the two case studies and the results suggest that it is a cost-effective strategy that would fit into many practical situations.