Meta-analysis of esp studies, 1987-2010: Assessing the success of the forced-choice design in parapsychology

Abstract

We report the results of a meta-analysis on forced-choice ESP studies which used targets such as card symbols, numbers, letters, and so forth. For the period 1987 to 2010, a homogeneous dataset of 72 forced-choice studies yielded a weak but significant mean effect size (ES) of 0.01 (Stouffer Z = 4.86, p = 5.90 × 10 -7). There was no evidence that these results were due to low-quality design or selective reporting. The clairvoyance studies did not produce a significantly higher mean ES than the precognition studies, and target type did not make a difference to effect size. We note that effects do not vary between investigators, but we did find suggestive evidence that the number of choices per trial is inversely related to the p value. We also found evidence of a linear incline in ES values indicating that effect sizes have increased over the period 1987 to 2010. Suggestions are made that might help facilitate further increases in effect sizes.