Interpreting the magnitude of the placebo effect: mountain or Molehill?

Abstract

The ambiguity involved in interpreting numbers and words is central to Hróbjartsson and Gøtzsche's (this issue) claim of "powerful spin" in the Wampold, Minami, Tierney, Baskin, and Bhati (2005) re-analysis of their meta-analytic findings on the placebo effect in medicine. Meta-analytic results reported by the two sets of authors are nearly identical, yet their conclusions differ dramatically. In our comment, we discuss the findings of the respective authors and consider options for representing and interpreting the magnitude of meta-analytic effect size estimates. We conclude that although the meta-analyses described indicate that placebo effects do exist and cannot be dismissed as unimportant, given contextual information, it is consistent with existing research to describe the obtained mean effect size for placebos in medicine as small in magnitude.