NO.PZ2018062016000143
问题如下：
Claire paid close attention to two portfolios specializing in stocks. The two portfolios were nearly in risk(measured by standard deviation of return) during 2013-2017. Claire believed that the mean monthly return difference between the two portfolio equal to zero and gathered the data of mean monthly retun of two portfolios from 2013-2017. In order to test the hypothesis that the mean monthly return on two portfolios equal from 2013-2017, which of the following test statistic is most appropriate?
选项：
A.a paired comparisons t-test.
B.an approximate t-test of mean differences between the two populations.
C.a t-test of the difference between the two population means.
解释：
A is correct. The two portfolios have the same series of risk factors, so their returns were not independent. Besides, the data is arranged in paired observations. We choose the paired comparisons t-test.
risk相近说明方差相似，不能说明他们是dependent或者independent吧？
