Fault coupling is the phenomenon whereby a test set is able to detect faults when they occur in isolation, but fails to do so when they occur in combination. It is widely regarded as a nuisance in fault-based approaches to software testing, which focus on the detection of single faults and normally neglect multiple faults.
This paper presents a theoretical study of fault coupling, based on a simple model of faultbased testing. This provides for the presence of two faults that interact with each other and thus includes the possibility of fault coupling between them. The model is analysed mathematically, the conclusion reached being that fault coupling only occurs infrequently. This result provides support for current approaches to fault-based testing, but it is not quite enough to conclude that they are thereby validated. In effect, the paper generalizes the results of a previous paper that dealt with the restricted case where the functions underlying programs are bijective as well as finite. The restriction that functions be bijective is lifted here, but they are still required to be finite. Though the same theoretical framework is used in both cases, and more or less the same results are obtained, the techniques employed to arrive at the results in the general case are quite different.