The unappreciated heterogeneity of effect sizes: Implications for power, precision, planning of research, and replication.

Repeated investigations of the same phenomenon typically yield effect sizes that vary more than one would expect from sampling error alone. Such variation is even found in exact replication studies, suggesting that it is not only because of identifiable moderators but also to subtler random variation across studies. Such heterogeneity of effect sizes is typically ignored, with unfortunate consequences. We consider its implications for power...