Why You Can’t Volunteer for Election Polls, and Other Polling Facts

There is no way to increase your odds of being chosen for one of Ms. Selzer’s polls, which have been among the most accurate in the country in past elections. They would be less reliable if there were.

Because demographics like race, sex, age and education level are associated with differences in political opinion, polls are more accurate when the demographic breakdown is accurate. If, for example, women or white people or baby boomers are over- or underrepresented in a sample, the poll results are likely to be skewed.

Pollsters address that problem through a process called weighting. If, say, college-educated men account for a larger percentage of the sample than they do of the full electorate, the pollster will count their responses a little less in the results.

In high-quality polls, these adjustments tend to be small, because a properly selected sample will have a close-to-accurate representation of most demographics to begin with. So a reputable organization would take a sample that was 54 percent women and weight it to match the percentage shown in the census, but a sample that was 75 percent women would be fatally flawed.

“If your original sample is way off on a certain demographic, no amount of weighting is going to help you,” said Doug Schwartz, director of the Quinnipiac University Poll. But if you choose your sample properly, he added, “you’re not going to be way off on any of the demographics.”

Many people are familiar with the margin of sampling error, which reflects the uncertainty involved in taking a sample of a larger population. A margin of error of plus or minus three percentage points means that a candidate’s actual support could easily be as much as three percentage points higher or lower than what the poll says.

That margin is tied to something called a confidence interval. Most polls publish a 95 percent confidence interval: Five percent of the time, that interval won’t contain the truth because of sampling error. That rate also may not capture every possible polling problem, like people not picking up the phone.

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