Skewed polls: How One 19-Year-Old Illinois Man Is Distorting National Polling Averages

There is a 19-year-old black man in Illinois who has no idea of the role he is playing in this election.

He is sure he is going to vote for Donald J. Trump.

And he has been held up as proof by conservatives — including outlets like Breitbart News and The New York Post — that Mr. Trump is excelling among black voters. He has even played a modest role in shifting entire polling aggregates, like the Real Clear Politics average, toward Mr. Trump.

How? He’s a panelist on the U.S.C. Dornsife/Los Angeles Times Daybreak poll, which has emerged as the biggest polling outlier of the presidential campaign. Despite falling behind by double digits in some national surveys, Mr. Trump has generally led in the U.S.C./LAT poll. He held the lead for a full month until Wednesday, when Hillary Clinton took a nominal lead.

Our Trump-supporting friend in Illinois is a surprisingly big part of the reason. In some polls, he’s weighted as much as 30 times more than the average respondent, and as much as 300 times more than the least-weighted respondent.

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Not all of the poll’s choices were bound to help Mr. Trump. But some were, and it all combined with some very bad luck to produce one of the most persistent outliers in recent elections.

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But the U.S.C./LAT poll is a panel — which means it recontacts the same voters over and over — and so it wound up with the worst of both worlds.

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But the U.S.C./LAT poll had terrible luck: The single most overweighted person in the survey was unrepresentative of his demographic group. The people running the poll basically got stuck at the extreme of the added variance.