The Placebo Control Group

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Abstract
This article reports a review and meta-analysis of placebo control groups in educational experiments. Studies were clustered into control procedures that included explicit manipulation of attentional elements (Attn + ), explicit description of the placebo activity plus implied reference to attention (Act+ ), and exclusive reference to the activity (Act-Only). The Act + procedure was the practice most consistent with the model for a placebo control. Compared with the other two procedures, Act+ controls were employed more often in interpersonal rather than academic studies in which subjects knew they were participating in an experiment, had an expectancy for performance change, and the placebo activity resembled the treatment task. Significantly larger effect sizes in Act + studies were found with the absence of experimenter controls and child subjects. The results were compared with findings from Hawthorne-labeled control groups in educational studies, and the differences between placebo-labeled control groups in education and those used in psychotherapy research were considered.