问题如下：
3. Based on the historical record of surprises in inflation and productivity, the historical equity risk premium for the US equity market, if it is used asan estimate of the forward-looking equity risk premium, should most likely be:
选项：
A.left unchanged.
B.adjusted upward.
C.adjusted downward.
解释：
C is correct.
A string of favorable inflation and productivity surprises may result in a series of high returns that increase the historical mean estimate of the equity risk premium. To mitigate that concern, the analyst may adjust the historical estimate downward based on an independent forward-looking estimate.
我们为什么要估计历史的ERP，因为我们要对股票进行估值，估值就需要折现率。那么分析不能凭空捏造一个ERP出来，所以他们首先根据历史情况计算出一个ERP，然后再根据未来经济走势对这个ERP进行调整。
这是meggie老师的解答，但是我觉着ERP这里总有点怎么说都有理的感觉：
如果根据历史，经济好了，Rm高，那么ERP当然就高了，所以本题以forward-looking的角度来看，ERP是向下调整。但是结合李老师上课讲的，按照历史经验得出，ERP是逆周期的，即经济好了，ERP是下降了，因为经济好所以风险低，和本题的答案又有点悖论。有点拿捏不准应该以哪个为准？
