In exercises that call for two-sample t procedures, you may use either of the two approximations for the degrees of freedom that we have discussed: the value given by your software or the smaller of n1−1 and n2−1. Be sure to state clearly which approximation you have used.
7.72 Pooled equals unpooled?
The software outputs in Figure 7.10 (pages 387–388) give the same value for the pooled and unpooled t statistics. Do some simple algebra to show that this is always true when the two sample sizes n1 and n2 are the same. In other cases, the two t statistics usually differ.