Since the statistical conclusion of a hypothesis test relies on sample data, there are two types of errors that may occur: a Type I error or a Type II error. A Type I error is committed when we reject the null hypothesis when it is actually true. On the other hand, a Type II error is made when we do not reject the null hypothesis when it is actually false.

W e denote the probability o f a T ype I erro r b y α and the probability o f a Type II error by β .

For a given sample size n, a decrease (increase) in α will increase (decrease) β . However, both α and β will decrease if the sample size n increases.