Selecation Bias

Statistical error that causes a bias in the sampling portion of an experiment. The error causes one sampling group to be selected more often than other groups included in the experiment.

This may produce an inaccurate conclusion if the selection bias is not identified. For example, if an experiment only selects people of a certain race with similar characteristics and excludes any group that deviates from these elements.

Undercoverage occurs when some members of the population are inadequately represented in the sample. A classic example of undercoverage is the Literary Digest voter survey, which predicted that Alfred Landon would beat Franklin Roosevelt in the 1936 presidential election. The survey sample suffered from undercoverage of low-income voters, who tended to be Democrats. Undercoverage is often a problem with convenience samples .

Undercoverage. Undercoverage occurs when some members of the population are inadequately represented in the sample

Voluntary response bias occurs when sample members are self-selected volunteers, as in voluntary samples . An example would be call-in radio shows that solicit audience participation in surveys on controversial topics (abortion, affirmative action, gun control, etc.). The resulting sample tends to overrepresent individuals who have strong opinions.

Sometimes, individuals chosen for the sample are unwilling or unable to participate in the survey. This can be a big problem with mail surveys, where the response rate can be very low.

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

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