Ever since Donald Trump has risen to the top of the polls, Republican strategists have questioned whether those polls might be overestimating his support.

There is evidence to support that theory.

We wrote about Mr. Trump’s support on Sunday, using polling data provided by Civis Analytics, a Democratic data firm founded by the 2012 Obama campaign’s analytics director, Dan Wagner. It showed Mr. Trump faring worse than in many recent public polls, but it was not enough to call his lead into question.

An analysis of the data revealed that he fares best among voters who don’t regularly participate in primary elections. Nevertheless, he still leads the other G.O.P. candidates, even among the most frequent voters.

The analysis also suggests that although Mr. Trump fares better among irregular voters, it’s not by an unusual amount. And some other candidates also do better with infrequent voters.

Before delving into the results, it’s important to understand why Civis polls are unlike the surveys sponsored by news organizations or universities. Nearly all public polls try to interview adults by randomly calling telephone numbers, a technique known as random digit dialing. They adjust the responses to match the demographic characteristics of the adult population, then remove those people who say they’re not registered to vote.

The technique is well grounded in statistical theory, but it has shortcomings for election polling. One of those shortcomings is that the adult population doesn’t necessarily reflect the electorate, and that’s especially true in primaries. Most adults, after all, do not vote in primary elections. People tend to self-report that they’re voters, even when they’re not.

The public polls can be misleading if nonvoting adults have substantially different views from primary voters. Usually, the difference isn’t huge. But some analysts have argued that Mr. Trump’s celebrity might give him an unusual advantage among irregular voters, who might not pay much attention to political news.

The candidates’ campaigns, for whom more accurate results really matter, have moved to a different approach to polling: They draw their sample from voter registration files, which contain information on voting history. That gives campaigns a powerful tool for examining the likely electorate.

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.

Civis Results Differ From Polling Averages

The Civis poll showed far more undecided voters and fewer supporters of Donald Trump.

The number of undecided voters can affect the rest of the results in a survey. For instance, if undecided voters were allocated proportionally until the number of undecided voters fell to the level of other polls, Mr. Trump would hold 21 percent of the vote. It’s not a good idea to assume that undecided voters break proportionally, but it gives a sense of how much undecided votes could explain the difference between the Civis data and the public polls. (Mr. Trump leads in a national polling average, at 26 percent).

The huge gap between the Civis results for Mr. Trump and other surveys also hints at the possibility that these voters, pushed to make a choice in public polls, might be breaking for Mr. Trump by a disproportionate margin.

At the very least, the large number of undecided voters suggests that many who tell pollsters they support a candidate might have weak preferences, and that Mr. Trump might be the beneficiary.

The large number of undecided voters makes it difficult to directly compare the Civis numbers with other public polls. But the Civis analysis, using voter history data, nonetheless offers good reason to believe that Mr. Trump might fare best among voters who have little history of voting.

Civis tried different methods to measure the effect of voter participation. All the methods showed the same basic story, with Mr. Trump faring better among irregular voters, but not by an unusual amount.

It was also able to look at the impact on other candidates. Jeb Bush, Ben Carson and Chris Christie fared better among irregular voters by a similar or even greater amount. Carly Fiorina, Scott Walker, Mike Huckabee, Ted Cruz and John Kasich fared better among more regular primary voters than irregular voters.

Who Fares Better Among Primary Voters?

Levels of support changed among some candidates when polling data was weighted to reflect primary voters, rather than all Republican adults.

Civis used a simple model to estimate the relationship between candidate preference and vote history — measured by the total number of elections respondents had voted in since 2000. It found that Mr. Trump, Mr. Bush and Mr. Carson all did much worse among voters who had participated in more elections.

Mrs. Fiorina saw her support rise with vote participation, as did Mr. Walker, Mr. Cruz, Mr. Kasich and Mr. Huckabee.

Civis took another step and weighted its sample under two scenarios: one reflecting the adult population of Republicans, the other of primary voters. Primary voters aren’t just likelier to vote; they’re also older and likelier to be registered Republicans. Mr. Trump fared two percentage points worse among primary voters than Civis estimated he would have among all adults. Other candidates, like Mr. Bush and Mr. Carson, actually lost more ground. Like Mr. Trump, Mr. Bush also seems to be benefiting from name recognition.

On the other hand, Mrs. Fiorina gained around three points in the primary electorate — although she still held no more than 5 percent — while Mr. Kasich, Mr. Walker, Mr. Huckabee and Rand Paul each picked up about a point.

Parsing the results by vote history helps illustrate that Mr. Trump’s support was lowest among the most frequent voters. Mr. Trump had 15 percent support among voters who had participated in a primary since 2008, but he had 22 percent of the vote among Republicans who did not vote in the 2012 general election.

Mr. Trump’s seven-point gap was rivaled by Mr. Bush at five, and Chris Christie at four. Mr. Christie had virtually no support — at 1 percent of the vote — among voters who had voted in a primary since 2008.

Graphic Who’s Winning the G.O.P. Campaign? Polls get most of the attention, but at this early stage, endorsements and fund-raising are more important.