Increasing Sample Sizes in Psychology over Time

Psychological studies often suffer from low statistical power due to small sample sizes. In this preregistered study, we updated the study by Marszalek et al. (2011) to examine whether sample sizes in psychology have increased over time. We collected data from 3176 studies across six journals over three years. Results show a significant increase in sample sizes over time (b=44.83, t(6.25)=4.48, p=.004, 95%CI[25.23,64.43]), with median sample sizes being 40 in 1995, 56.5 in 2006, and 122.5 in 2019. This growth appears to be a response to the credibility crisis. While open science practices at the study level did not significantly impact sample sizes, we found that journals with stronger open science policies reported larger samples (b=42.94, t(3172)=5.98, p<.001, 95%CI[28.83,57.05]). These results suggest that journal-level policy can play a crucial role. The increase in sample sizes is a promising development for the replicability and credibility of psychological science.