Pooled Standard Deviation

Pooled standard deviation is the standard deviation remaining after removing the effect of special cause variation-such as geographic location or time of year. It is the average variation of your subgroups.

Pooled Standard Deviation combines two such deviations to then compare their statistical difference.

s = sqrt[((n1-1)s1^2 + (n2-1)s2^2)/(n1+n2-2)]