NO.PZ2017092702000122
问题如下：
An analyst is examining a large sample with an unknown population variance. Which of the following is the most appropriate test to test the hypothesis that the historical average return on an index is less than or equal to 6%?
选项：
A. One-tailed z-test
B. Two-tailed z-test
C. One-tailed F-test
解释：
A is correct.
If the population sampled has unknown variance and the sample is large, a z-test may be used. Hypotheses involving "greater than" or "less than" postulations are one-sided (one-tailed). In this situation, the null and alternative hypotheses are stated as H0: μ ≤ 6% and Ha: μ > 6%, respectively. A one-tailed t-test is also acceptable in this case, and the rejection region is on the right side of the distribution.
检验历史的平均收益是否小于等于6%，这样的假设就是单尾的一个概念
且对于检验总体均值而言，（总体）方差已知用z，（总体）方差未知用t，非正态（总体）小样本不可估计。
所以选择单尾&z-test
已知不才用z吗？未知不是用t吗
