When you download today’s list of S&P 500 members and test a strategy over the last twenty years, you are quietly testing only the winners — the companies that survived long enough to still be in the index. Companies that went bankrupt, were acquired or fell out of the index disappear from the test entirely.
Why it matters
Studies of U.S. equity data have shown that ignoring delisted stocks can noticeably overstate the historical returns of simple stock-selection strategies, especially those that buy cheap or beaten-down companies — exactly the kind of companies most likely to be delisted.
How to avoid it
- Use point-in-time index membership, not today’s list.
- Include delisted securities and their final prices.
- Record why each security left the dataset (merger, bankruptcy, exchange move).
All DataBacktest equity datasets include delisted symbols by default.
Important: Backtested results are hypothetical and are prepared with the benefit of hindsight. They do not represent actual trading and do not guarantee future performance. DataBacktest does not provide investment advice. See our Risk Disclaimer.
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