What is 'Sample Selection Bias'
Sample selection bias is a type of bias caused by choosing nonrandom data for statistical analysis. The bias exists due to a flaw in the sample selection process, where a subset of the data is systematically excluded due to a particular attribute. The exclusion of the subset can influence the statistical significance of the test, or produce distorted results.
BREAKING DOWN 'Sample Selection Bias'
Survivorship bias is a common type of sample selection bias. For example, when backtesting 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 15year 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 15year period, our final results would be flawed, as these performed well enough to survive the market.

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A subset containing the characteristics of a larger population. ... 
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Central Limit Theorem  CLT
A statistical theory that states that given a sufficiently large ...

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