问题如下：
The country Y has established its own equity index in 2011. The index vendor constructed returns for the five years prior to 2011 based on the initial group of companies constituting the index in 2011. What would be the effect on historical equity risk premium of including index returns prior to 2011?
选项：
A. It has no effect on the historical equity risk premium estimate.
B. It would bias the historical equity risk premium estimate upwards.
C. It would bias the historical equity risk premium estimate downwards.
解释：
B is correct.
考点：Survivorship bias
解析：B是正确的。这道题考察的是存活偏误，能够存活下来的公司都是业绩好的公司，回填指数构建日期之前的公司收益会产生存活偏差使得历史股票风险溢价被高估。
回填什么意思？是指用现在的Rm，来估计历史的Rm吗？
