Relating Code Coverage, Mutation Score and Test Suite Reducibility to Defect Density

Assessing the overall quality (adequacy for a particular purpose) of existing test suites is a complex task. Their code coverage is a simple yet powerful attribute for this purpose, so the additional benefits of mutation analysis may not always justify the comparably much higher costs and complexity of the computation. Mutation testing methods and tools slowly start to reach a maturity level at which their use in everyday industrial practice becomes possible, yet it is still not completely clear in which situations they provide additional insights into various quality attributes of the test suites. This paper reports on an experiment conducted on four open source systems' test suites to compare them from the viewpoints of code coverage, mutation score and test suite reducibility (the amount test adequacy is degraded in a reduced test suite). The purpose of the comparison is to find out when the different attributes provide additional insights with respect to defect density, a separately computed attribute for the estimation of real faults. We demonstrate that in some situations code coverage might be a sufficient indicator of the expected defect density, but mutation and reducibility are better in most of the cases.