The 4PL(Click on "Preview" below the photo at the left to see inside.) This book explains the mathematical form of the four parameter logistic model that is the basis of many current bioassays. You'll acquire an understanding of how relative potency is estimated by comparing two 4PL models. Youll learn what probits are, why so called probit analysis was once necessary to estimate relative potency, why probit analysis (and "logit analysis") became known as