Look-ahead bias

Using information in a trading decision that would not have been available at the moment the decision was actually made. The classic case is computing a signal from a bar’s closing price and then booking the fill at that same bar’s open, when the close has not yet happened. The tell is a result that is too good; the fix is ensuring every value a decision touches is knowable strictly before the decision’s timestamp, usually via a .shift(1).

First used in Lesson 2.5 · The bias catalog: hunting the ways a backtest lies — the lesson that makes this term real.