Look-Ahead and Survivorship Bias
Leakage is any path by which information from the future reaches a model that is supposed to be making decisions in the past. Every form of it inflates backtest performance, because the model is being graded on questions it has partially seen the answers to. The two oldest and most commonly tested forms are look-ahead bias and survivorship bias.
Look-ahead bias
A backtest commits look-ahead bias when a decision at time uses data that was not knowable at time .
The mechanism is usually mundane rather than exotic: a dataset timestamps a quarterly earnings figure to the quarter it describes rather than the day it was announced, or a "daily" fundamental field is populated with values that were later restated. The strategy appears to react to news before the market because, in the data, the news arrives early.
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