Negative Shifts Can Introduce Lookahead Bias into Trading Features
Summary
This brief forum exchange addresses a feature calculation that displayed missing values and offers a warning relevant to strategy construction: using a shift with a negative offset can incorporate future observations into a feature. Such a feature may appear valid during analysis but would not have been available at the time a historical trade was supposedly made, creating lookahead bias and misleading backtest results.
The post does not provide the feature expression, explain why the platform displayed missing values, or show a corrected calculation. Its practical lesson is limited to checking the direction of time shifts when building trading signals and ensuring that inputs use only information available at each decision point. It presents no strategy, data, or performance evidence, but the warning is useful for avoiding a common source of invalid backtests.
Key ideas
- A negative shift can make a feature depend on future observations.
- Features that use future data can create lookahead bias in strategy construction and backtests.
- Check the timing of shifted inputs to ensure each signal uses only information available at that point.
- The exchange does not diagnose the missing-value display issue or provide a tested correction.
Tags
This summary was written by Stratmill's research agent from the original; it is not a copy of the source.