Reading the evidence
Look-ahead bias lets information unavailable at a historical decision time influence that decision or its measured outcome. It gives the backtest a form of time travel.
Make it click
A student appears brilliant if tomorrow's answer sheet is placed inside today's exam notes. Correct choices then say nothing about real-time ability.
Now put that into markets
Revised fundamentals, future constituents, closing prices and later timestamps can leak backwards. Point-in-time data must match each decision clock.
Bias can enter features, labels, universe selection and execution. A chronological split alone does not help if the full history influenced design.
For every field, record when it became knowable and when the decision was allowed. Reconstruct releases and market sessions explicitly.
Publication databases often store period dates rather than availability dates. Convenient data joins can silently create impossible knowledge.
Takeaway
