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Investopedia explains 'Sample Selection Bias'
Survivorship bias is a common type of sample selection bias. For example, when back-testing an investment strategy on a large group of stocks, it may be convenient to look for securities that have data for the entire sample period. If we were going to test the strategy against 15 years worth of stock data, we might be inclined to look for stocks that have complete information for the entire 15-year period. However, eliminating a stock that stopped trading, or shortly left the market, would input a bias in our data sample. Since we are only including stocks that lasted the 15-year period, our final results would be flawed, as these performed well enough to survive the market.
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