The P Value: What It Is and What It Is Not

The P value remains one of the most frequently reported statistical measures in biomedical literature, yet it is also one of the most widely misunderstood statistics. Introduced by Fisher as a measure of evidence against the null hypothesis, it was subsequently incorporated into the Neyman-Pearson decision framework, which emphasized long-run error rates and decision thresholds. Over the past decades, reliance on the conventional cut-off of P = 0.05 has fostered misconceptions, including the belief that the P value represents the probability that the null hypothesis is true or that statistical significance implies clinical importance. In this review, I examine the historical evolution of the P value, clarify the conceptual distinctions between evidential and decision-theoretic perspectives, and illustrate their implications through a case study. Common misinterpretations and the limitations of threshold-based inference are discussed, together with the consequences for reproducibility, statistical power, and interpretation of results. Recent recommendations from statistical associations and methodologists to move beyond dichotomous significance testing are highlighted. Complementary approaches, such as estimation of effect sizes with confidence intervals (CIs), likelihood ratios, and Bayesian inference, are briefly considered. I conclude that although the P value may provide useful information when properly interpreted, it should not be used as a sole criterion for inference. Transparent reporting of effect sizes, CIs, and contextual information offers a more reliable foundation for scientific interpretation and decision making.