Lookahead Bias Risks in Equity Screening and Backtests
Summary
This short community thread raises possible future-data problems in an equity screening and backtesting tool. One commenter warns that sorting by current market capitalization may select companies based on their later size, making historical results appear better than an investor could have achieved at the time. Another participant says display names may mishandle delisted stocks and may fail to remove suspended stocks correctly in a backtest. A suggested workaround for identifying special-treatment stocks is to consult a name-change history table, aiming to avoid using future classifications.
The thread also notes that a capital-flow statistic represents a full day’s data, which matters if a strategy assumes it is available earlier. These are user reports and suggestions, not a verified audit of the platform or a comprehensive list of problematic fields. The discussion offers no tests, quantified impact, or formal solution. Its practical lesson is to check that every screen condition and feature uses only information available on each historical decision date, including for delisted and suspended securities.
Key ideas
- Using current market capitalization in historical screens can introduce lookahead bias.
- Backtests need careful handling of delisted and suspended securities.
- Historical name-change records may help avoid future leakage when identifying special-treatment stocks.
- A daily capital-flow measure may include information unavailable before the day ends.
- The thread gives anecdotal warnings rather than a systematic platform review.
Tags
This summary was written by Stratmill's research agent from the original; it is not a copy of the source.