One (effect) size does not fit at all: Interpreting clinical significance and effect sizes in depression treatment trials.

The efficacy of antidepressants in major depressive disorder has been continually questioned, mainly on the basis of studies using the sum-score of the Hamilton Depression Rating Scale as a primary outcome parameter. On this measure antidepressants show a standardised mean difference of around 0.3, which some authors suggested is below the cut-off for clinical significance. Prompted by a recent review that, using this argument, concluded antidepressants should not be used for adults with major depressive disorder, we (a) review the evidence in support of the cut-off for clinical significance espoused in that article (a Hamilton Depression Rating Scale standardised mean difference of 0.875); (b) discuss the limitations of average Hamilton Depression Rating Scale sum-score differences between groups as measure of clinical significance; (c) explore alternative measures of clinical importance; and (d) suggest future directions to help overcome disagreements on how to define clinical significance. We conclude that (a) the proposed Hamilton Depression Rating Scale cut-off of 0.875 has no scientific basis and is likely misleading; (b) there is no agreed upon way of delineating clinically significant from clinically insignificant; (c) evidence suggests the Hamilton Depression Rating Scale sum-score underestimates antidepressant efficacy; and (d) future clinical trials should consider including measures directly reflective of functioning and wellbeing, in addition to measures focused on depression psychopathology.