How To Calculate Pooled Estimate Of Population Variance. Pooled variance (also called combined, composite, or overall variance) is a way to estimate common variance when you believe that different populations have the same variances. The formula to calculate a pooled standard deviation for two groups is as follows:

The pooled variance estimates the population variance (σ 2) by aggregating the variances obtained from two or more samples. S p 2 = σ i = 1 k ( n i − 1) s i 2 / σ i = 1 k ( n i − 1) again, if all groups have the same number of observations, then the formula simplifies to σ i = 1 k s i 2 / k, which is the average of the k variances. V p = (n1 − 1)s2 1 + (n2 −1)s2 2 n1 + n2 −2.