P-curve

Last updated on Feb 26, 2026
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Definition: P-curve is a tool for identifying potential publication bias and makes use of the distribution of significant p-values in a series of independent findings. The deviation from the expected right-skewed distribution can be used to assess the existence and degree of publication bias: if the curve is right-skewed, there are more low, highly significant p-values, reflecting an underlying true effect. If the curve is left-skewed, there are many barely significant results just under the 0.05-threshold. This suggests that the studies lack evidential value and may be underpinned by questionable research practices (QRPs; e.g., p-hacking). In the case of no true effect present (true null hypothesis) and unbiased p-value reporting, the p-curve should be a flat, horizontal line, representing the typical distribution of p-values.

Related terms: File-drawer, Hypothesis, *P*\-hacking, *p*\-value, Publication bias (File Drawer Problem), Questionable Research Practices or Questionable Reporting Practices (QRPs), Selective reporting, Z-curve

References:

  • Bruns, S. B., & Ioannidis, J. P. (2016). P-curve and p-hacking in observational research. PLoS ONE, 11(2), e0149144. https://doi.org/10.1371/journal.pone.0149144
  • Simonsohn, U., Nelson, L. D., & Simmons, J. P. (2014). P-curve: a key to the file-drawer. Journal of Experimental Psychology: General, 143(2), 534. https://doi.org/10.1037/a0030850
  • Simonsohn, U., Nelson, L. D., & Simmons, J. P. (2014). P-curve and effect size: Correcting for publication bias using only significant results. Perspectives on Psychological Science, 9(6), 666–681. https://doi.org/10.1177/1745691614553988
  • Simonsohn, U., Nelson, L. D., & Simmons, J. P. (2019). P-curve won’t do your laundry, but it will distinguish replicable from non-replicable findings in observational research: Comment on Bruns & Ioannidis (2016). PLoS ONE, 14(3), e0213454. https://doi.org/10.1371/journal.pone.0213454

Originally drafted by: Bettina M. J. Kern

Reviewed by: Sam Guay, Kamil Izydorczak, Charlotte R. Pennington, Robert M. Ross, Olmo van den Akker