Pollsters refer to this phenomenon as the “shy Trump” effect, or — in academic parlance — a form of “social-desirability bias.” Studies have affirmed that in races where a candidate or cause is perceived as controversial or otherwise undesirable, voters can be wary of voicing their support, especially to a live interviewer .

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Charles Franklin, the director of the Marquette University Law School poll of Wisconsin voters, said he worried that the shy Trump effect had played a role in skewing the poll’s results away from Mr. Trump in 2016.

Mr. Franklin, who was a member of the AAPOR team, suggested how telephone interviewers might confront the issue with respondents next year: “When they indicate they’re undecided or maybe considering a third-party vote, maybe push people a little more on whether they could change their mind,” he said.

One polling firm that showed Mr. Trump narrowly leading in some of the most inaccurately polled states — Michigan, Pennsylvania and Florida, all of which he won — was Trafalgar Group, a Republican polling and consulting firm that uses a variety of nontraditional polling methodologies.

It sought to combat the shy Trump effect by asking respondents not only how they planned to vote but also how they thought their neighbors would vote — possibly offering Trump supporters a way to project their feelings onto someone else .

The AAPOR report posited that the neighbor question could help overcome shyness among Trump supporters, particularly in phone interviews. It “warrants experimentation in a broad array of contests,” the report said.

Who’s voting?

That was not the only way Trafalgar innovated. Polls typically use a formula based on past elections to determine which voters are likely to show up on Election Day. They then discard or devalue responses from those who seem less predisposed — typically those without much history of voting, or who don’t express much enthusiasm about politics.