the error of failing to reject the null hypothesis when it is in fact not true. Investigators make this error if they conclude that a particular effect or relationship does not exist when in fact it does. The probability of committing a Type II error is called the beta (β) level of a test. Conversely, the probability of not committing a Type II error (i.e., of detecting a genuinely significant difference between samples) is called the power of the test, where power = 1 – β. Also called beta error.