t-testtable.htm. Table of critical values of. t. : One Tailed Significance level: df: Two Tailed Significance level: 0.2. 0.1. What critical value should be compared to the t value obtained as the test statistic? Solution: The sample size is 12. Hence the degrees of freedom ( df) = 12 - 1 = 11. Mapping the alpha level across the row for one-tailed alpha values and the df of 17 on the left-side of the table, we get our critical value as 1.363. When is T Distribution used? In one tailed t-tests, the critical value of t from t-distribution table represents the rejection area of distribution either left or right of the mean. In single tailed t-test, the critical value of t at a specified level of significance (α) is calculated either left side or right side of the mean of t-distribution. Q6i5Ko.