Software-testing researchers have long sought recipes for test suites that detect faults well. In the literature, empirical studies of testing techniques abound, yet the ideal technique for detecting the desired kinds of faults in a given situation often remains unclear. This work shows how understanding the context in which testing occurs, in terms of factors likely to influence fault detection, can make evaluations of testing techniques more readily applicable to new situations. We present a methodology for discovering which factors do statistically affect fault detection, and we perform an experiment with a set of test-suite- and fault-related factors in the GUI testing of two fielded, open-source applications. Statement coverage and GUI-event coverage are found to be statistically related to the likelihood of detecting certain kinds of faults.