The critical values are fixed values which can be obtained from the standard t crit tables. We can find the critical value tc when the degrees of freedom and confidence level are given. The degrees of freedom is simply:
Degrees of Freedom = Number of samples – 1
Degrees of Freedom = 19 – 1
Degrees of Freedom = 18
From the t critical tables, the tc corresponding to Degrees of Freedom equal to 18 and Confidence level equal to 95% is as follows:
tc = 2.101
Therefore this means that any t value beyond 2.101, we already reject the null hypothesis.