Dr. Brenda Masters

When comparing two means a choice needs to be made about whether the variances of the two populations are equal.   To compare two means, first decide can an assumption be made about the equality of the two population variances.   If the two populations can be assumed equal then the two variance estimates can be combined to construct a better variance estimate with higher degree of freedom.  This variance estimate is known as a pooled variance estimate.   Complete notes on this procedure and on the situation when equality of variances is not assumed is found at Comparing two means .