look-ahead bias

Look-ahead bias is the phenomenon that unconsciously uses unavailable or unrevealed data in analyzing or simulating historical events. It exists in the processes of making decisions or evaluations which use information or data that was unknown at that time.  Look-ahead bias may cause distortion and misleadingness of the result because it violates the principle of using only information available during analysis. It could emerge in any field, such as finance, economy, and data analysis, and influence investment strategy, backtesting of the trading system, and performance grading.