On P values and effect modification

A crucial element of evidence-based healthcare is the sound understanding and use of statistics. As part of instilling sound statistical knowledge and practice, it seems useful to highlight instances of unsound statistical reasoning or practice, not merely in captious or vitriolic spirit, but rather, to use such error as a springboard for edification by giving tangibility to the concepts at hand and highlighting the importance of avoiding such error. This article aims to provide an instructive overview of two key statistical concepts: effect modification and P values.

A recent article published in the Journal of the American College of Cardiology on side effects related to statin therapy offers a notable example of errors in understanding effect modification and P values, and although not so critical as to entirely invalidate the article, the errors still demand considerable scrutiny and correction. In doing so, this article serves as an instructive overview of the statistical concepts of effect modification and P values.

Judicious handling of statistics is imperative to avoid muddying their utility. This article contributes to the body of literature aiming to improve the use of statistics, which in turn will help facilitate evidence appraisal, synthesis, translation, and application.