Although researchers often assume their participants are naive to experimental materials, this is not always the case. We investigated how prior exposure to a task affects subsequent experimental results. Participants in this study completed the same set of 12 experimental tasks at two points in time, first as a part of the Many Labs replication project and again a few days, a week, or a month later. Effect sizes were markedly lower in the second wave than in the first. The reduction was most pronounced when participants were assigned to a different condition in the second wave. We discuss the methodological implications of these findings.

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Keywords effect sizes, judgment and decision making, nonnaïveté, open data, open materials, panel conditioning, repeated participation, research methods
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Journal Psychological Science
Chandler, J, Paolacci, G, Peer, E, Mueller, P, & Ratliff, K.A. (2015). Using Nonnaive Participants Can Reduce Effect Sizes. Psychological Science, 26(7), 1131–1139. doi:10.1177/0956797615585115