The objective of an equivalence study is to demonstrate if the two drug products are equivalent. However, “equivalence” may be defined in various ways. It may mean that the two drugs lead to similar responses; or to similar efficacies (over placebo) within a defined margin of equivalence. Depending on which of the two objectives, margin determination, study design, measurement and test may be different. In a study with only teat and reference products or treatments, the equivalence measure may be difference in means or ratio of means. Equivalence of two products/treatments is then demonstrated by showing that the measurement of difference is between the pre-specified lower and upper margins of equivalence. Therefore, the null hypothesis of interest is that the difference measurement is either lower than the lower margin or larger than the upper margin. In another word, one may demonstrate equivalence by showing that the difference is laying between the two margins. The complete test may be performed with two sets of hypotheses. In a three products/treatment (placebo, test and reference) study, in addition to the equivalence test, the assay sensitivity of reference is established by showing reference is superior to placebo; efficacy of test is demonstrated by showing test is superior to placebo. Equivalence test is used in in-vivo studies for equivalence assessment of a generic product or a post-market change of a new product. It is also used in in-vitro studies for equivalence assessment of chemical or physical parameters. It is also used in clinical trials for equivalence assessment of patient treatment responses of the two products.
