For a single independent variable, such as a dosage level, the models for the probabilities can be justified on the basis of a population with mean and scale parameter of tolerances for the subjects. Then, given a dose x, the probability, , of observing a response in a particular subject is the probability that the subject’s tolerance is less than the dose or

Thus, in this case, the intercept parameter, , and the regression parameter, , are related to and by

Note:The parameter is not equal to the standard deviation of the population of tolerances for the logistic and extreme value distributions.