For some surveys, it is important to ensure that there are enough members of a certain subgroup in the population so that more reliable estimates can be reported for that group. To do this, we oversample members of the subgroup by selecting more people from this group than would typically be done if everyone in the sample had an equal chance of being selected. Because the margin of sampling error is related to the size of the sample, increasing the sample size for a particular subgroup through the use of oversampling allows for estimates to be made with a smaller margin of error. A survey that includes an oversample weights the results so that members in the oversampled group are weighted to their actual proportion in the population; this allows for the overall survey results to represent both the national population and the oversampled subgroup.
For example, African Americans make up 13.6% of the total U.S. population, according to the U.S. Census. A survey with a sample size of 1,000 would only include approximately 136 African Americans. The margin of sampling error for African Americans then would be around 10.5 percentage points, resulting in estimates that could fall within a 21-point range, which is often too imprecise for many detailed analysis surveyors want to perform. In contrast, oversampling African Americans so that there are roughly 500 interviews completed with people in this group reduces the margin of sampling error to about 5.5 percentage points and improves the reliability of estimates that can be made. Unless a listed sample is available or people can be selected from prior surveys, oversampling a particular group usually involves incurring the additional costs associated with screening for eligible respondents.
An alternative to oversampling certain groups is to increase the overall sample size for the survey. This option is especially desirable if there are multiple groups of interest that would need to be oversampled. However, this approach often increases costs because the overall number of completed interviews needs to be increased substantially to improve the representation of the subgroup(s) of interest.
The studies reported below included an oversample in the sampling design. Among other groups, these studies included oversamples of young people, African Americans, internet users, and parents of young children. See the “About the Survey” section in each report for details.
- Millennials: Confident. Connected. Open to Change. February 24, 2010 (oversample of 18 to 29 year olds)
- Blacks Upbeat about Black Progress, Prospects: A Year After Obama’s Election January 12, 2010 (oversample of African Americans and Hispanics)
- Likely Rise in Voter Turnout Bodes Well for Democrats July 10, 2008 (oversample of 18 to 29 year olds)
- Inside the Middle Class: Bad Times Hit the Good Life April 9, 2008 (oversample of African Americans and Hispanics)
- Blacks See Growing Values Gap Between Poor and Middle Class November 13, 2007 (oversample of African Americans)
- Motherhood Today: Tougher Challenges, Less Success May 2, 2007 (oversamples of African Americans, Hispanics, and 18 to 49 year olds)
- Most Say Imus’s Punishment Was Appropriate April 18, 2007 (oversample of African Americans)
- Broad Support for Political Compromise in Washington January 22, 2007 (oversample of congressional districts that switched party control in the 2006 election)
- A Portrait of “Generation Next” January 9, 2007 (oversample of 18 to 25 year olds)
- Lack of Competition in Elections Fails to Stir Public October 27, 2006 (oversample of congressional districts with competitive races for U.S. House seats in the 2006 election)
- Democrats Hold Double Digit-Lead in Competitive Districts October 26, 2006 (oversample of congressional districts with competitive races for U.S. House seats in the 2006 election)
- The Black and White of Public Opinion October 31, 2005 (oversample of African Americans)
- Two-In-Three Critical of Bush’s Relief Efforts September 8, 2005 (oversample of African Americans)
- The 2004 Political Landscape November 5, 2003 (oversample of African Americans)
- Faith-Based Funding Backed, But Church-State Doubts Abound April 10, 2001 (oversample of African Americans)
- Interdiction and Incarceration Still Top Remedies March 21, 2001 (oversample of African Americans)
- The Internet News Audience Goes Ordinary January 14, 1999 (oversample of internet users)
- GOP Congressional Lead Not Undercut by Backlash October 21, 1998 (oversample of congressional districts with competitive races for U.S. House seats in the 1998 election)
- White House Scandal Has Families Talking September 30, 1998 (oversample of parents)
- Voters Not So Angry, Not So Interested June 15, 1998 (oversample of California residents)
- How Americans View Government March 10, 1998 (oversample of African Americans)
- Americans Going Online… Explosive Growth, Uncertain Destinations October 16, 1995 (oversample of internet users)
- The People, the Press & Politics September 21, 1994 (oversample of African Americans)
- The Vocal Minority in American Politics July 16, 1993 (oversample of radio talk show callers)
- Campaign ’92: Survey VIII July 8, 1992 (oversample of young people aged 18 to 34 and African Americans)