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American Association for Public Opinion Research

Measuring Sub-Group Preferences

Sub-Groups: Size Matters

Soccer Moms, NASCAR Dads, Evangelical Christians, the Internet Married – we know that political strategists like to group voters into categories in order to target their candidate’s message. But without oversampling, election polls are limited in uncovering the vote intention or opinions of these microgroups because of their small size.

Journalists need to take into account that when subgroup results are reported, the margin of sampling error for those figures is larger than the sampling error for results based on the sample as a whole. (see graph on this page) Journalists should identify the number of respondents in the subgroup, even for larger groups such as Democrats or men or Hispanics or senior citizens.

The following table shows the number of respondents in various subgroups from a statewide poll of a battleground state in the Midwest.

 
*Full range of sampling error accounts for the fact that the margin of sampling error is typically reported as plus or minus a single value. When taken together the result is the full range.

Sample sizes below 100 will have a large margin of sampling error – plus or minus 10 percentage points for a sample size of 100 and increasing as the sample size declines. In line with standard practice at many polling firms, journalists may want to avoid reporting on groups this small unless there is a compelling reason to do so, and even then only after consulting with an independent polling expert.

This information was developed by AAPOR as part of a comprehensive online journalism polling course created in partnership with NewsU, a project of the Poynter Institute and funded by the Knight Foundation. The course launched  September 2007.

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