Redundant mutants, where multiple mutants end up producing same the semantic variant of the program is a major problem in mutation analysis, and a measure of effectiveness is an essential tool for evaluating mutation tools, new operators, and reduction techniques. Previous research suggests using size of disjoint mutant set as an effectiveness measure.
We start from a simple premise: That test suites need to be judged on both the number of unique variations in specifications they detect (as variation measure), and also on how good they are in detecting harder to find bugs (as a measure of subtlety). Hence, any set of mutants should to be judged on how best they allow these measurements.
We show that the disjoint mutant set has two major inadequacies — the single variant assumption and the large test suite assumption when used as a measure of effectiveness in variation, which stems from its reliance on minimal test suites, and we show that when used to emulate hard to find bugs (as a measure of subtlety), it discards useful mutants.
We propose two alternative measures, one oriented toward the measure of effectiveness in variation and not vulnerable to either single variant assumption, or to large test suite assumption and the other towards effectiveness in subtlety, and provide a benchmark of these measures using diverse tools.