Detecting Evidential Value and P-Hacking With the P-curve tool: A Word of Caution

Simonsohn, Nelson, and Simmons (2014a) proposed p-curve – the distribution of statistically significant p-values for a set of studies – as a tool to assess the evidential value of these studies. They argued that, whereas right-skewed p-curves indicate true underlying effects, left-skewed p-curves indicate selective reporting of significant results when there is no true effect (“p-hacking”). We first review previous research showing that, in contrast to the first claim, null effects may produce right-skewed p-curves under some conditions. We then question the second claim by showing that not only selective reporting but also selective nonreporting of significant results due to a significant outcome of a more popular alternative test of the same hypothesis may produce left-skewed p-curves, even if all studies reflect true effects. Hence, just as right-skewed p-curves do not necessarily imply evidential value, left-skewed p-curves do not necessarily imply p-hacking and absence of true effects in the studies involved.