The quantity Φ−1(p), where p is a proportion or probability, and Φ is the cumulative distribution function of the standard normal distribution. When it is believed that the probability of success, p, is dependent upon the values of some predictor variables, it is usual to work with some function of p that has an infinite range in order to ensure that the estimate of p lies in the interval (0, 1). One such function is the logit; another is the probit.