Equal or unequal variances
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If you check Assume equal variances, the sample standard deviations are pooled to obtain a single estimate of s.

The two-sample t-test with a pooled variances is slightly more powerful than the two-sample t-test with unequal variances, but serious error can result if the variances are not equal. Therefore, the pooled variance estimate should only be used when a hypothesis test suggests your variances are equal. Use Test for Equal Variances to test the equal variance assumption.