An approximate approach to sampling size determination for the equivalence hypothesis

The equivalence hypothesis is the correct hypothesis to confirm whether the new test product conforms to the standard reference product. It has many applications to evaluation of generic drug products and other new clinical modalities. The two one-sided tests (TOST) procedure was proposed to test the equivalence hypothesis for two treatments. When the difference in population means between two treatments is not 0, the proportion of the type II error rate allocated to each of the two tails of the central t-distribution cannot be analytically determined. Hence, no close form of the exact sample size for the equivalence hypothesis is available. Currently only approximate formulas are proposed. The resulting sample sizes may provide either insufficient power or unnecessarily excessive power. We suggest an approximate approach with consideration of type II error rates for both one-sided hypotheses to determination of the sample size for the equivalence hypothesis. The results of a numerical study are reported. Remarks on the usage of different methods for the sample size determination for the equivalence hypothesis in practical applications are provided.