The pooled standard deviation is a method for estimating a single standard deviation to represent all independent samples or groups in your study when they are assumed to come from populations with a common standard deviation.

What is the pooled standard deviation and when is it used?

A pooled standard deviation is simply a weighted average of standard deviations from two or more independent groups. In statistics it appears most often in the two sample t-test, which is used to test whether or not the means of two populations are equal.

How do you know when to use pooled variance?

The pooled variance is a better estimate of the (unknown) common group variance than either of the individual group variances. If each group has the same number of observations, then the pooled variance is a simple average.