Our first task is to determine the degrees of freedom (df ), which approximates the sample size (but, for fancy technical reasons, adjusts it slightly to make for a more accurate outcome). For this particular test statistic, the degrees of freedom is n1 − 1 + n2 − 1 or n1 + n2 − 2 (putting the terms in either order results in the same value). So for each group, add the size of the two samples and subtract 2. In this example, 30 + 30 − 2 = 58. This is the degrees of freedom for this particular application of the t test but not necessarily for any other.
