Look-ahead bias

Also seen as: Lookahead bias, information leakage, peeking

What causes look-ahead bias in a backtest?

Look-ahead bias appears whenever the date a value is stamped with differs from the date it became public, and the test uses the stamp. The recurring sources are:

• Publication lag on reported fundamentals. A quarterly figure describes a quarter that ended weeks or months before the filing appeared. Attaching that figure to the quarter-end date lets a decision act on it before anyone could have read it.
• Restatements and revisions. Many series are revised after first release. Using the final, revised value as though it had been known on the first-release date imports information that did not exist yet.
• Same-moment execution. Deciding on a day's closing price and then filling at that same day's open, or using a session's high or low before it had printed, gives the fill a price the decision could not have known. Which price a fill uses is set by the execution time.
• Today's membership applied to yesterday. Running a test on the constituents of an index as it stands now over a period when the membership was different mixes look-ahead with survivorship bias.

Worked example: earnings used before they were published

A screen ranks companies on their latest reported earnings and rebalances on 1 February. Company A's fiscal quarter ended 31 December, but its results were only published on 20 February. A test that files the December figure under 31 December can see it on 1 February and buys on it; a test that files it under 20 February cannot see it yet and buys something else. Repeat that across every filing for ten years and the strategy has enjoyed a three-week head start on every earnings surprise — a systematic advantage no real account had. This is why a filter lag counted in Reports Ago rather than calendar months matters: filings, not the calendar, decide when a figure was knowable. See lag, period, and multiplier for how those units work.

How does walk-forward replay avoid look-ahead bias?

Walk-forward replay reduces look-ahead bias by moving through history one step at a time and letting each decision use only what had already happened by that step. Fincanva backtests replay your rules in chronological order and run to the latest available market close, never past it, so a decision dated in 2015 cannot draw on 2016 data. What that removes is the computation's opportunity to look ahead; what it cannot remove is a rule that reads a figure the market did not yet have on that date, which is why the verb here is reduce and not eliminate. See walk-forward replay for the mechanic and the nine biases Fincanva helps you avoid for the product behavior behind it.

How is look-ahead bias different from the other backtest biases?

Look-ahead bias is about when the information was available; its neighbours are about what you tested and how you read the result. Survivorship bias is a sample missing the names that did not last. Selection bias is a sample chosen with hindsight. Data-snooping bias is many tests with only the winner reported. Overfitting is a strategy shaped so tightly to past data that it captures noise. Data-quality bias is the nearest neighbour of all: a conclusion driven by the stored data itself being wrong, which is where restatements and revisions land when the corrected value is the only one you have. A test can be entirely free of look-ahead bias and still suffer every one of the others.

Backtests show what would have happened — not what will. Fincanva provides no financial advice — see Is this financial advice?.