Statisticians use the Greek letter β (beta) to indicate the probability of failing to reject the hypothesis tested when that hypothesis is false and a specific alternative hypothesis is true. For a given test, the value of β is determined by the previously selected value of α (alpha), certain features of the statistic that is being calculated (particularly the sample size) and the specific alternative hypothesis that is being entertained.

While it is possible to carry out a statistical test without entertaining a specific alternative hypothesis, neither β nor power can be calculated if there is no specific alternative hypothesis. It is relevant to note here that power (the probability that the test will reject the hypothesis tested when a specific alternative hypothesis is true) is always equal to one minus β (i.e. Power = 1 - β).