NO.PZ2018062016000113
问题如下：
When sampling size is 50 with mean is 78, and population variance is unknown, which of the following distribution is taken if sampling forms a normal distribution?
选项：
A. z-statistic only
B. both t or z statistic
C. t-statistic only
解释：
B is correct, when sampling form a normal distribution with unknown variance, if sample size is larger than 30, both t or z-statistic is choose.
当样本形成方差未知的正态分布时，若样本量大于 30，则选择 t 或 z 统计量。
解析中，怎么断定样本量大于30呢？ 题目中给的是sample size，意思是取样时的数量，n的意思不是取样的次数吗？也就是sample1，sample2…sample50
