NO.PZ2018062016000141
问题如下：
Which of the following statements on a pooled estimator most appropriate?
选项：
A.It's an equality of the variances of two normally distributed populations.
B.It's an estimate drawn from the combination of two different samples.
C.It can only be used in Chi-squared test
解释：
B is correct. A pooled estimate is an estimate drawn from the combination of two different samples. It's used when testing the concerning difference between means.
我所理解的是当衡量different between means with independent samples 时, 如果 variance not equal 用S1, S2, 如果 variance equal 时用pooled estimator.
请问为何 "an equality of the variances of two normally distributed populations" 不是 pooled estimator? 谢谢