If Mr. Trump had a big advantage among unlikely voters, a poll using a listed sample — like the Civis data — would be the way to find out. The Civis poll was conducted Aug. 10 to 19 and had a sample of 757 respondents. That sample was as much as three times larger than that of some public polls.

The results showed Mr. Trump with 16 percent of the vote, below any of his poll results in a month. But much of the difference was because 22 percent of voters in the Civis poll were undecided — much more than in many recent public surveys.

The proportion of respondents who “don’t know” often varies from pollster to pollster, a phenomenon called a “house effect.” That’s because the wording of the questions varies or the interviewers have been trained to push undecided voters into making a choice. Sometimes it depends on whether pollsters asked all respondents or only those registered to vote.