  In order to determine whether or not an effect is absent based on a
statistical test, the recommended frequentist tool is the equivalence test.
Typically, it is expected that an appropriate equivalence margin has been
specified before any data are observed. Unfortunately, this can be a difficult
task. If the margin is too small, then the test's power will be substantially
reduced. If the margin is too large, any claims of equivalence will be
meaningless. Moreover, it remains unclear how defining the margin afterwards
will bias one's results. In this short article, we consider a series of
hypothetical scenarios in which the margin is defined post-hoc or is otherwise
considered controversial. We also review a number of relevant, potentially
problematic actual studies from clinical trials research, with the aim of
motivating a critical discussion as to what is acceptable and desirable in the
reporting and interpretation of equivalence tests.
