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When to trust the polls

With the election just around the corner and the political polling season in full swing, a U-M researcher's advice can help voters tell good numbers from bad.

"Surveys draw on two human propensities that have served us well from ancient times," says Howard Schuman, Institute for Social Research (ISR) senior research scientist emeritus. "One is to gather information by asking questions. The other is to learn about one's environment by examining a small part of itwhich is the basis of sampling."

Schuman, a sociologist, wrote about the issue in the summer issue of Contexts magazine, in an article titled "Sense and Nonsense About Surveys."

The value of a sample, he says, comes not only from its size but also from the way it was obtained. In its simplest form, the modern technique of probability sampling calls for each person in the population to have an equal chance of being selected. As a result, most Internet surveys do not come close to representing the general population adequately, Schuman says, since not everyone has access to the Internet.

Surprisingly, the size of the sample that's needed depends little on the size of the population, he says. "For example, almost the same size sample is needed to estimate the proportion of left-handed people in the United States as to make the same estimate for, say, Peoria, Ill.," Schuman says. "In both cases, a reasonably accurate estimate can be obtained with a sample size of around 1,000."

The margin of error plus/minus figures featured in many survey and poll reports reflect the size of the sample, but don't reveal how many people refused to answer or could not be contacted. This non-response rate can affect the poll results if those missed differ from those who are interviewed on the issues being studied, Schuman says. So paying attention to the survey response rate, as well as the margin of error, makes good sense when deciding whether to trust a poll.

"In some federal surveys, the percentage of non-response is small, within the range of five to 10 percent," he says. "But for even the best non-government surveys, the refusal rate can reach 25 percent or more, and it can be far larger in the case of poorly executed surveys."

With answering machines making direct contacts with people harder, and annoyance with telemarketers leading to higher rates of refusal, rates of non-response have increased in recent years. For example, the number of calls needed to complete a single interview in the monthly ISR Survey of Consumers doubled between 1979 and 1996.

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