Mutant Quality Indicators

The question of which are the valuable mutants has received little attention in mutation testing literature. Naturally, the choice of mutants impacts the quality of the performed analysis and has the potential of changing the conclusions of empirical studies. To this end, we collect definitions related to mutant quality indicators and analyse their relations. We identify two classes of indicators, related to individual mutants (Fault Revealing, Subsuming, Hard-to-kill and Stubborn) and to mutant sets (disjoint/dominator and distinguished). We analyse a large set of mutants from 3,902 (real) faulty program versions, belonging to 40 fault classes, collected from an on-line programming contest. Our analysis categorises mutants as valuable, according to the studied quality indicators, profiles their types and examines the relations between them. Our results suggest that there is a large disagreement between the indicators and that the connection between mutant type, its quality and its ability to reveal faults is weak. Additionally, our paper reveals that the ability of mutants to uncover faults differs significantly across the different fault classes and that some mutant types are well linked (or completely disconnected) to specific fault classes.