P values: use and misuse in medical literature.

P values are widely used in the medical literature but many authors, reviewers, and readers are unfamiliar with a valid definition of a P value, let alone how to interpret one correctly. Popular explanations such as "the probability that study results are due to chance" are wrong in a variety of ways and can lead to substantial errors in evaluating the evidence from research studies. Belief that "statistical significance" can alone discriminate between truth and falsehood borders on magical thinking. The article points out how to better interpret P values by avoiding common errors. Statistical analyses and P values are important tools in evidence-based medicine, but have to be used cautiously and with better understanding.