Core terms
Look-ahead bias
Using information in a test or decision that was not available at the decision time, such as full-day volume for a morning entry or a fill at a bar's low.
In intraday backtests, look-ahead hides in ordinary fields: full-day volume, the day's high, low or close, and lists ranked by full-day change used as the universe for early entries. Fills are another source, such as an entry at a 1-minute bar's low or a stop filled at its exact price. Labels such as "closed green" or "first red day" need the session's end.
A simple check: write the time at which each filter's value was first known; if it is later than the entry, the filter is look-ahead. The FennEdge Gapper HOD study splits gappers by whether they closed at or above the open (n = 233, median high of day 10:25 ET) or below it (n = 438, median 08:37 ET) and flags that the split uses after-close information (2026-07-01 to 2026-10-02; FennEdge archive; as of 2026-10-02). It describes two kinds of day; as a filter for a 09:35 entry it would be look-ahead.
What traders should verify
Next to every filter in a backtest or plan, write the time its value was first known, and model fills at the next available price rather than at a bar's extreme.
Do not conflate
Trader usage; there is no official definition. The term is also common in backtesting research. Look-ahead usually makes a rule look better than anyone could have traded it; a rule that only works with it is a description of past days.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.