TradeHouse Fidelity Lab · Guides
Look-ahead bias in backtests
Look-ahead bias is any use of information in a backtest that would not have been available when the trading decision was made. It is the most damaging form of contamination because it can make a strategy with no edge look excellent, and it is often invisible in the equity curve.
Common causes
- Same-bar fills. A signal computed from a bar's close and filled at that same close. In live trading the close is not known until the bar ends.
- Revised data. Fundamentals or economic figures loaded as later restated, rather than as first published.
- Current index membership. Testing on today's index constituents over past years. This mixes look-ahead with survivorship bias.
- Timestamp misalignment. Data in different time zones, or end-of-period stamps treated as start-of-period.
- Normalisation over the full sample. Z-scores or scalers fitted on the whole history, including the future.
- Outcome-stamped features. In prediction markets, a record created after resolution used as an entry signal.
How to detect it
- For each feature, write down the time it becomes known and compare it with the decision time.
- Delay all fills by one bar. A large performance drop points to timing leakage.
- Check for features whose correlation with the outcome is implausibly high.
- Compare backtest fills with the quotes that were actually available at the stated time.
How Fidelity Lab flags it
Look-ahead is a BLOCKER-severity detector in Fidelity Lab. It flags outcomes or features stamped before entry, and records created after the outcome they predict. A related detector, oracle lag, flags settled win rates far above the price paid, the signature of a stale quote scored as a fill. The report names the specific legs, so you can fix the data at the source and re-run.
Try it on your own trades. The free Fidelity Lab tier runs one report a month on up to 5,000 legs: all 11 contamination detectors, cost re-netting and the walk-forward verdict. Run a free report or see pricing.
Frequently asked questions
Is look-ahead bias the same as overfitting?
No. Overfitting comes from selecting among many trials. Look-ahead bias comes from information leaking from the future. A backtest can have either, or both.
Can look-ahead bias exist in a strategy with no parameters?
Yes. It is about timing of information, not the number of parameters.
Related guides
- Backtest overfitting: a practical guide
- Walk-forward validation vs cross-validation
- Backtest validation tool: what it should check
- Probability of backtest overfitting (PBO)
- Trading strategy audit service
- Overfitting in backtrader
- Trading strategy robustness tests
- Monte Carlo backtests
- Survivorship bias in backtests
- Data snooping in trading
- Out-of-sample testing in trading
- What is backtest overfitting?
- Deflated Sharpe ratio calculator
- Fidelity Lab
Educational content, not investment advice. Updated 2026-10-28.