1.6 Type I, Type II Errors, Statistical Power

Type I error is the probability of rejecting the null hypothesis when the null hypothesis is true (false-positive). The probability of committing a type I error is called \(\alpha\). If a study is designed with \(\alpha\) = 0.05, then the researcher has set 5% as the maximum chance of incorrectly rejecting the null hypothesis. Type II error is the probability of failing to reject the null hypothesis when the null hypothesis is false (false-negative). The probability of committing a type II error is called \(\beta\). The quantity (\(1-\beta\)) is called power, the probability of correctly rejecting the null hypothesis (lower right corner).

Truth in the population	Accept null hypothesis	Reject null hypothesis
Null hypothesis is true	Correct	Type I error
Null hypothesis is false	Type II error	Correct