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The sample size varies with each organization and institution due to the way the data are processed. Gallup often interviews only five hundred respondents, while Rasmussen Reports and Pew Research often interview one thousand to fifteen hundred respondents.
Rasmussen Reports. 2015. “Daily Presidential Tracking Poll.”
Rasmussen Reports . September 27, 2015. http://www.rasmussenreports.com/public_content/politics/obama_administration/daily_presidential_tracking_poll (February 17, 2016); Pew Research Center. 2015. “Sampling.”
Pew Research Center . http://www.pewresearch.org/methodology/u-s-survey-research/sampling/ (February 17, 2016). Academic organizations, like the American National Election Studies, have interviews with over twenty-five-hundred respondents.
American National Election Studies Data Center. 2016. http://electionstudies.org/studypages/download/datacenter_all_NoData.php (February 17, 2016). A larger sample makes a poll more accurate, because it will have relatively fewer unusual responses and be more representative of the actual population. Pollsters do not interview more respondents than necessary, however. Increasing the number of respondents will increase the accuracy of the poll, but once the poll has enough respondents to be representative, increases in accuracy become minor and are not cost-effective.
Michael W. Link and Robert W. Oldendick. 1997. “Good” Polls / “Bad” Polls—How Can You Tell? Ten Tips for Consumers of Survey Research.”
South Carolina Policy Forum . http://www.ipspr.sc.edu/publication/Link.htm (February 17, 2016); Pew Research Center. 2015. “Sampling.”
Pew Research Center . http://www.pewresearch.org/methodology/u-s-survey-research/sampling/ (February 17, 2016).
When the sample represents the actual population, the poll’s accuracy will be reflected in a lower margin of error. The margin of error is a number that states how far the poll results may be from the actual opinion of the total population of citizens. The lower the margin of error, the more predictive the poll. Large margins of error are problematic. For example, if a poll that claims Hillary Clinton is likely to win 30 percent of the vote in the 2016 New York Democratic primary has a margin of error of +/-6, it tells us that Clinton may receive as little as 24 percent of the vote (30 – 6) or as much as 36 percent (30 + 6). A lower of margin of error is clearly desirable because it gives us the most precise picture of what people actually think or will do.
With many polls out there, how do you know whether a poll is a good poll and accurately predicts what a group believes? First, look for the numbers. Polling companies include the margin of error, polling dates, number of respondents, and population sampled to show their scientific reliability. Was the poll recently taken? Is the question clear and unbiased? Was the number of respondents high enough to predict the population? Is the margin of error small? It is worth looking for this valuable information when you interpret poll results. While most polling agencies strive to create quality polls, other organizations want fast results and may prioritize immediate numbers over random and representative samples. For example, instant polling is often used by news networks to quickly assess how well candidates are performing in a debate.
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