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Empirical assessment of published effect sizes and power in the recent cognitive neuroscience and psychology literature

Fig 2

Degrees of freedom and effect size (D-value) distribution in the literature.

The histograms are zoomed in for better visibility, but all data were used in calculations. (A) Extracted df distribution in all 26,841 records. (The distributions were nearly overlapping for statistically significant and nonsignificant results.) (B) The distribution of D-values in statistically significant (p ≤ 0.05) and nonsignificant (p > 0.05) records in the whole data set (“computed”) and in the subset of data with D-value reports (“reported”). (C) The bivariate distribution of D-values and degrees of freedom in the whole data set. The density of statistical records is color coded, as shown by the calibration bar on the right. Curves in the figure are described in a left-to-right order. The leftmost dashed curve shows the expected value of effect sizes if the null hypothesis is true. The dotted curve shows the mean effect size from nonsignificant records in the data (the median was nearly the same). The middle thick continuous red curve denotes the significance threshold with p ≤ α where α = 0.05, in terms of D-values. The dotted-dashed blue curve and the rightmost continuous thin blue curve show the median and mean effect sizes only for statistically significant effect sizes, for various degrees of freedom.

Fig 2