NO.PZ2017092702000141
问题如下：
A pooled estimator is used when testing a hypothesis concerning the:
选项：
A.equality of the variances of two normally distributed populations.
B.difference between the means of two at least approximately normally distributed populations with unknown but assumed equal variances.
C.difference between the means of two at least approximately normally distributed populations with unknown and assumed unequal variances.
解释：
B is correct.
The assumption that the variances are equal allows for the combining of both samples to obtain a pooled estimate of the common variance.
如下图所示，这是一个POOLED ESTIMATOR。
之所以为POOLED，可以理解为在等式右边需要同时用到S1以及S2两个不同样本的方差。而上图的公式，是T检验中一步，这个T检验就是检验B选项说论述的假设检验。
B C but and 一词之差，可以解释下表达意思有什么不同吗？
