The nature of fault exposure ratio

The fault exposure ratio K is an important factor that controls the per-fault hazard rate, and hence the effectiveness of software testing. The paper examines the variations of K with fault density which declines with testing time. Because faults get harder to find, K should decline if testing is strictly random. However, it is shown that at lower fault densities K tends to increase, suggesting that real testing is more efficient than random testing. Data sets from several different projects are analyzed. Models for the two factors controlling K are suggested, which jointly lead to the logarithmic model