ABSTRACT Mean effect sizes for changes in depression were calculated for 2,318
patients who had been randomly assigned to either antidepressant medication or placebo in
19 double-blind clinical trials. As a proportion of the drug response, the placebo
response was constant across different types of medication (75%), and the correlation
between placebo effect and drug effect was .90. These data indicate that virtually all of
the variation in drug effect size was due to the placebo characteristics of the studies.
The effect size for active medications that are not regarded to be antidepressants was as
large as that for those classified as antidepressants, and in both cases, the inactive
placebos produced improvement that was 75% of the effect of the active drug. These data
raise the possibility that the apparent drug effect (25% of the drug response) is actually
an active placebo effect. Examination of pre-post effect sizes among depressed individuals
assigned to no-treatment or wait-list control groups suggest that approximately one
quarter of the drug response is due to the administration of an active medication, one
half is a placebo effect, and the remaining quarter is due to other nonspecific factors.
