How are critical values determined

The critical values are determined so that the probability that the test statistic has a value in the rejection region of the test when the null hypothesis is true equals the significance level (denoted as α or alpha).

Explanation:

Determine the critical value by finding the value of the known distribution of the test statistic such that the probability of making a Type I error — which is denoted (greek letter "alpha") and is called the "significance level of the test" — is small (typically 0.01, 0.05, or 0.10).