Engineers use a variety of test solutions to help identify intermittent signals - the key metric is probability of intercept (POI). The choice of technique can dramatically affect the detection of signals that are important in radar, EW, EMI, military communications and satellite communications. As a result, understanding POI for an instrument is a complex challenge. This paper will address the key variables in the analysis of such signals as well as the techniques used to optimize instrumentation to find rare signal events. It will also compare techniques that range from traditional swept-tuned spectrum analysis to new capabilities that rely on advanced signal processing.