Science, timing figure into polls
Why do political polls so often diverge, from slightly to significantly? I know they sometimes narrow their results by age, gender, race, party, but even within those groups we get very different results between Gallup and Pew. Do the different pollsters target other different demographics?
For an authoritative answer, we put your question to Ellen Drizin, Times senior customer insight analyst. She responds:
"You ask a good question on a complex issue. Gallup, Pew and other responsible pollsters take care to follow best practices, such as randomly selecting participants, using clear, unbiased wording, weighting results to reflect the characteristics of registered voters, and interviewing a large number of respondents to minimize the margin of error.
"But even with these precautions, you're still bound to see differences in poll results. Some of this can be explained by differences in the margin of error or weighting methods. But the timing of a poll is also critical. Election polls are sometimes called 'horse race' polls, so the timing of where we are in that race is especially important.
"For example, on Oct. 8, Gallup published two poll results on the same day, one in the morning showing a dead heat between Mitt Romney and Barack Obama, and another in the afternoon showing Obama leading by 5 percentage points. On the same day, Pew released a study showing Romney leading by 4 points among likely voters.
"The Pew and morning Gallup poll results were based mainly on interviews conducted over several days following Romney's strong performance in the Oct. 3 presidential debate, while the afternoon Gallup report was based on a seven-day rolling average that included both pre-debate and post-debate results.
"Beyond traditional measures, pollsters must also take into consideration and adjust for newer challenges such as early voting and the growth of cellphone-only households.
"Given all of the variables that go into polling, what's a poll watcher to do? First, realize that polls conducted by responsible pollsters are still the most scientific and objective way to measure current attitudes and opinions. Looking at how the most dependable polls average out often leads to accurate predictions. RealClearPolitics.com and the Huffington Post's Huffpost Politics Election Dashboard both offer websites that trend results averaged from recent polls.
"It may also be a comfort to know that poll predictions have become increasingly accurate. According to data compiled by the National Council on Public Polls (NCPP), the accuracy of predictions based on poll averages has steadily increased since 1936, with both 2004 and 2008 predictions coming within one percentage point of the actual outcome. With many expecting a tight election and hard battles being fought in swing states, it will be interesting to see how close final poll predictions come this year."