Significance Level

The significance level (also called Type I error rate or the level of statistical significance) refers to the probability of rejecting a null hypothesis that is in fact true. This quantity ranges from zero (0.0) to one (1.0) and is typically denoted by the Greek letter alpha (a). The significance level is sometimes referred to as the probability of obtaining a result by chance alone. As this quantity represents an "error rate," lower values are generally preferred. In the literature, nominal values of a generally range from 0.05 to 0.10. The significance level is also referred to as the "size of the test" in that the magnitude of the significance level determines the end points of the critical or rejection region for hypothesis tests. As ...