A good experiment has 80% a priori chance of finding the effect, given that the effect is present at the population level. Many experiments in psychology are underpoweredĮver since Cohen’s ( 1962) classic study on statistical power, it has been known that many psychology studies contain too few observations to properly investigate the effects under examination. Our analyses can easily be applied to new datasets gathered.
We also show that researchers must include the number of observations in meta-analyses because the effect sizes currently reported depend on the number of stimuli presented to the participants. This is considerably more than current practice. On the basis of this analysis, we recommend that a properly powered reaction time experiment with repeated measures has at least 1,600 word observations per condition (e.g., 40 participants, 40 stimuli). We checked how we could estimate the power of each study and how much they could be reduced to remain powerful enough.
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To remedy the situation, we review the literature related to the topic and introduce recent software packages, which we apply to the data of two masked priming studies with high power. Researchers in cognitive psychology are hindered in estimating the power of their studies, because the designs they use present a sample of stimulus materials to a sample of participants, a situation not covered by most power formulas. For properly powered studies replication rate should be around 80%, whereas in practice less than 40% of the studies selected from different areas of psychology can be replicated. In psychology, attempts to replicate published findings are less successful than expected.
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