Statistics Dictionary
To see a definition, select a term from the dropdown text box below. The statistics
dictionary will display the definition, plus links to related web pages.
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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.
