P value

A p value is the probability of observing results at least as extreme as those reported in a study, assuming that the null hypothesis is true (usually ‘no difference’). Essentially, this is the probability of achieving the results by random chance. In a hypothesis test, this value is compared with the prespecified threshold for significance (α) to conclude whether there is evidence to reject the null hypothesis. Calculators are available to provide p values associated with study results expressed as Z scores (normal distribution), t-scores, chi-squared and many other distributions. A common misapprehension is that the p value supports reasoning about the probabilities of alternative hypotheses, whereas its use is primarily to decide whether or not to reject the null hypothesis.