In analog circuits, process variations result in physical parameter variations. Simulated values must then be considered with there tolerance intervals. Consequently, contrarily to digital circuits where the outputs are either '0' or '1' such that we can decide without ambiguity whether a fault is detectable or not, for analog circuits the fault detectability is a vague problem as the fault can either be completely detectable, partially detectable or completely undetectable which makes it very diOEcult to take a decision. In order to solve this decision problem, we have introduced the probability to detect fault (PDF) function which allows to formalize the problem of analog fault detection under parameter variations.