使用时间最长的数据

问题如下：

When Morgan considers the limitations of historical estimates, which of the following is most likely correct?

选项：

A.When many variables are considered, a larger number of observations is not a statistical necessity. B.The analyst should use the longest data history for which there is a reasonable assurance of stationarity. C.Using higher-frequency data (e.g., monthly, rather than annual observations) will provide more precise estimates, since this will improve the precision of the sample mean.

解释：

Answer Choice (A) is incorrect: when many variables are considered, a large number of observations may be a statistical necessity. Answer Choice (C) is also incorrect: although it is tempting to assume that using higher-frequency data (e.g., monthly rather than annual observations) will also provide more precise estimates, this assumption is not necessarily true. Although higher-frequency data improve the precision of sample variances, covariances and correlations, they do not improve the precision of the sample mean.