WASHINGTON (AP) — Voters in key swing states who settled on a candidate just before Election Day, along with polls that surveyed too few voters without a college degree, were among the culprits behind overconfidence that Hillary Clinton was sure to beat Donald Trump in the race for the White House, a new report suggests.
The analysis of 2016’s pre-election polls, released Thursday by the American Association for Public Opinion Research, found evidence that Trump voters were more likely than Clinton voters to reveal their preference to pollsters only after the election. But it’s unclear if that’s because they were shy about revealing their decision, or only made it in the final days of the campaign.
The report’s authors include widely respected survey researchers from a variety of backgrounds.
Pre-election polls suggested a close contest in many swing states, but the association’s report notes that state polls missed by a wider margin on average than they had in the four previous presidential elections. Polls showing Clinton ahead in Pennsylvania, Michigan and Wisconsin fueled perceptions that she had a “Blue Wall” in the Electoral College — a wall that collapsed on Election Day.
Although the report found fault with the accuracy of state polling, especially in the upper Midwest, it noted that national poll averages in the final two weeks of the campaign were among the most accurate in the last eight decades at estimating the national popular vote.
Among the factors the report suggests were responsible for the state polling miss:
LATE DECIDERS
Several states important to Trump’s victory had high proportions of voters who said they only settled on the Republican in the last days of the campaign. The report suggests that polls in those places may have been more accurate at the time they were conducted, before undecided voters came off the fence in support of the eventual winner.
In Michigan, Wisconsin, Pennsylvania and Florida, those who said they decided who to support in the final week of the campaign supported Trump by double-digit margins, according to exit polls conducted for The Associated Press and television networks by Edison Research.
The report found mixed evidence that this late wave in favor of Trump was directly tied to FBI Director James Comey’s announcement on Oct. 28 that the agency would review new evidence in its probe of Clinton’s use of a private email system as secretary of state. Her lead in national polls, researchers found, was already eroding at that time.
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SHY TRUMP VOTERS?
The report also found that about 1 in 10 Trump voters, but only about half as many Clinton voters, said before the election that they were undecided or supporting a different candidate, citing data from a Pew Research Center study in which respondents to pre-election surveys were re-interviewed after the election.
This finding could be consistent with the “shy Trump voter” theory, which says that supporters of the brash Republican were reluctant to admit their intention to vote for him because of social pressure.
Researchers found little other clear evidence for this hypothesis. The evidence also fits the more benign conclusion that many of Trump’s supporters decided or changed their minds shortly before they voted.
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EDUCATION
The report suggests that some pollsters may have failed to correct for an underrepresentation of voters without college degrees, who were particularly likely to support Trump and have long been less likely to respond to polls.
In general, some people are more likely to respond to polls than others, even if a pollster starts with a perfectly random sample. Nearly all surveys try to correct for this using procedures known as weighting, which attempt to make sure the survey includes the voices of the correct proportion of groups such as men and women, whites and racial minorities, and people in different age groups.
But particularly at the state level, not all pollsters weighted to correct for differing levels of education, which were more strongly correlated with vote choice in 2016 than in past elections. In some state polls that made full datasets were available, the report says weighting by education would have helped correct a pro-Clinton bias.
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TURNOUT
The report says some early analysis suggests the way pollsters estimated turnout and decided who was likely to vote may have also contributed to polling errors.
Some pollsters rely heavily on turnout from previous elections, either among individuals or demographic groups, to determine who is likely to vote in an upcoming election. Others rely more on self-reported intention to vote. In either instance, failing to correctly assess who will vote and who will not can lead to survey errors.
Predicting how turnout will change compared with previous elections is a fraught task for pollsters, and early evidence from 2016 suggests that turnout was down in some places among key Democratic voting blocs.
However, the report notes that some of the best estimates of turnout among individual demographic groups, including voter data from some states and a U.S. Census survey on voting and registration, are not yet available for 2016.
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Follow AP Polling Editor Emily Swanson on Twitter at: http://twitter.com/El_Swan
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