We evaluate the validity of the test of individual equivalence ratios
(TIER), a term coined by Anderson & Hauck (1990). The test was also pr
oposed in Wellek (1989), It is proved to be a valid test under a class
of unimodal symmetric distributions; this includes normal distributio
ns. Since most bioequivalence studies involve data which are normal, o
r at least unimodal and symmetric, the test is typically valid. It is,
however, not always valid, as shown by counter examples.