p-Hacking in A/B Testing

46 Pages Posted: 18 Jul 2018 Last revised: 21 Jul 2022

See all articles by Ron Berman

Ron Berman

University of Pennsylvania - The Wharton School

Leonid Pekelis


Aisling Scott


Christophe Van den Bulte

University of Pennsylvania - Marketing Department

Date Written: December 11, 2018


We investigate to what extent online A/B experimenters “p-hack” by stopping their experiments based on the p-value of the treatment effect, and how such behavior impacts the value of the experimental results. Our data contains 2,101 commercial experiments in which experimenters can track the magnitude and significance level of the effect every day of the experiment. We use a regression discontinuity design to detect the causal effect of reaching a particular p-value on stopping behavior.

Experimenters indeed p-hack, at times. Specifically, about 73% of experimenters stop the experiment just when a positive effect reaches 90% confidence. Also, approximately 75% of the effects are truly null. Improper optional stopping increases the false discovery rate (FDR) from 33% to 40% among experiments p-hacked at 90% confidence. Assuming that false discoveries cause experimenters to stop exploring for more effective treatments, we estimate the expected cost of a false discovery to be a loss of 1.95% in lift, which corresponds to the 76th percentile of observed lifts.

Keywords: A/B testing, p-hacking, false discoveries, false positives, experimentation

JEL Classification: C12, C90, C93, M21, M31

Suggested Citation

Berman, Ron and Pekelis, Leonid and Scott, Aisling and Van den Bulte, Christophe, p-Hacking in A/B Testing (December 11, 2018). Available at SSRN: https://ssrn.com/abstract=3204791 or http://dx.doi.org/10.2139/ssrn.3204791

Ron Berman (Contact Author)

University of Pennsylvania - The Wharton School ( email )

3641 Locust Walk
Philadelphia, PA 19104-6365
United States

Leonid Pekelis

OpenDoor ( email )

United States

Aisling Scott

Independent ( email )

Christophe Van den Bulte

University of Pennsylvania - Marketing Department ( email )

700 Jon M. Huntsman Hall
3730 Walnut Street
Philadelphia, PA 19104-6340
United States

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