Selection Bias

In survey sampling, the bias that results from an unrepresentative sample is called selection bias. Some common examples of selection bias are described below.

• Undercoverage. 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 .
  
  
• Voluntary response bias. 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.
  
  
• Nonresponse bias. 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.