What's the Value of the P Value?

Many studies published in current scientific journals include a P value as part of the data analysis, often as an indication of statistical significance of the results. What does the P value mean? A P value is related to the null hypothesis (i.e., there is no difference between the no-treatment and treatment groups) and represents the probability of obtaining the observed results if the null hypothesis is true (1). For example, a P value of 0.05 represents a 5% probability of obtaining the observed results if the null hypothesis (no difference) is true. In a recent Viewpoint article published in JAMA (2), John P.A. Ioannidis discusses a …