问题如下：
Nowacki’s
calculated price/book ratios (P/Bs) and price/earnings ratios (P/Es), in his
back-testing of the new strategy, are a problem because of:
选项：
A. data
mining.
B. look-ahead
bias.
C. survivorship
bias.
解释：
B is correct. Look-ahead bias results from using information that was unknown or unavailable at the time the investment decision was made. An example of this bias is using financial accounting data for a company at a point before the data were actually released by the company. Nowacki computed historical P/Bs and P/Es using calendar year-end (31 December) stock prices and companies’ financial statement data for the same calendar year, even though the financial statement data for that calendar year were likely unavailable at year-end.
Data mining refers to automated computational procedures for discovering patterns in large datasets, which can introduce a bias known as overfitting. Survivorship bias occurs when back-testing uses companies that are in business today but ignores companies that have left the investment universe.
请问回测数据又没有说用到将来，为什么是Look ahead呢？另外回测不应该有幸存偏误吗？factor里的成分券会变的吧？
