Abstract

A review of the basic concept of an analytic dilution assay—one in which relative potency is defined as the ratio of the quantity of effective constituent in unit amount of test to that in the standard—indicates that such an assay is rarely achieved in practice. It is proposed that there are no invalid assays, but only invalid inferences from a set of conditions in which relative potency has been determined and found variable, to new, unknown conditions. Nonparallel log dose-response lines are then interpreted to mean that relative potency varies among other things, with response level, where relative potency is defined as the ratio of doses leading to equal responses. Formulas are given for expressing log relative potency as a function of dose or level of response (formulas 3 and 4) and for making probability statements about it (formulas 9 and 10). The methods are illustrated by reference to a non-parallel log dose-response line assay of two different corticosteroids.