The two types of mistake that can be made when deciding whether or not to reject the null hypothesis. A type I error is rejecting a true null hypothesis; the probability of committing a type I error can be controlled by choosing the significance level of the test. A type II error is failing to reject a false null hypothesis; the probability of committing a type II error, in general, cannot be computed. The probability of not committing a type II error, that is, of correctly rejecting a false null hypothesis, is called the power of the test.