The pooled variance combines independent sample variances from groups assumed to share a common population variance. For 
groups with sample sizes 
and unbiased sample variances 
, it is
 |
Thus each group variance is weighted by its degrees of freedom rather than directly by its sample size.
The pooled variance is used in the equal-variance two-sample t-test and in one-way ANOVA. It is inappropriate when the population variances cannot reasonably be treated as equal.
