significance level
(symbol: α) in significance testing, a fixed probability of rejecting the null hypothesis of no effect when it is in fact true. It is set at some value, usually .001, .01, or .05, depending on the consequences associated with making a Type I error. When a particular effect is obtained experimentally, the probability level (p) associated with this effect is compared to the significance level. If the p value is less than the α level, the null hypothesis is rejected. Small p values suggest that obtaining a statistic as extreme as the one obtained is rare and thus the null hypothesis is unlikely to be true. The smaller the α level, the more convincing is the rejection of the null hypothesis. Also called alpha level.