The threshold at which the decision is made to reject the null hypothesis. The smallest significance level that is normally regarded as reasonable evidence is 5%, which means that the probability of a false positive (or Type I error) is less than 1 in 20. There is now considerable debate on whether choosing such an arbitrary critical value is helpful. Instead it is suggested one should quote the exact p-value, allowing the reader to assess the strength of evidence this suggests.

Defined on page 62

Used on pages 62, 63, 75, 76, 77, 80, 84, 98, 101