In statistical hypothesis testing (or tests of significance), one assumes that the null hypothesis is true about a reference population and attempts to reject it by seeking evidence for the alternative hypothesis. This is done by taking a sample and evaluating whether the sample provides evidence to support the alternative hypothesis. To do so, it is customary to compute the p value. The rejection of the null hypothesis depends on the comparison of the p value with a threshold probability value (chosen by the experimenter), which is referred to as the α level (or level of significance) of the test and is symbolized as the Greek letter α. This entry discusses the calculation and interpretation of the α level, the history of its use in ...