To take another example: What if the people who respond to telephone surveys are particularly politically engaged? Might the young voters who respond be more likely to be registered than young people in general — resulting in a sample of registered voters who are younger than in reality? It’s possible.

Voters could even misreport whether they’re actually registered to vote.

The Likely-Voter Screen

Even if a poll perfectly captures the opinion of registered voters, it might still miss the results if it makes an inaccurate forecast about who will vote and who will not.

Many mainstream media pollsters determine who is likely to vote with a slew of questions, like whether the respondents have voted before, whether they know where their precinct is or whether they’re enthusiastic. Some of these questions are of uncertain value. The Obama campaign pollsters, for instance, criticized the notion that people need to know where their precinct is to be considered likely voters. You can easily search for it on Google at the last minute.

Most pollsters then narrow the sample in proportion to the expected turnout: If the pollster believes 50 percent of adults will vote, then they might include the 50 percent of voters — the so-called cutoff — deemed likeliest to vote based on those questions. Often they choose a slightly higher cutoff than the expected turnout, on the assumption that poll respondents are likelier than nonrespondents to vote — but exactly how much likelier is somewhat unscientific.

The Selzer likely-voter screen is different and simple. The poll asks voters one question: whether they will vote. If they say they will “definitely” or “probably” vote, they’re a likely voter in the caucus. That’s it. Ms. Selzer makes no assumptions about the size of the electorate.

The advantage is that Ms. Selzer doesn’t need to make any judgments about turnout, which is notoriously hard to predict — especially in a caucus. While the range of plausible turnout in a general election is fairly narrow (say 54 percent of adults in 2000 to 62 percent in 2008), it is wide in the Iowa caucuses. In 2008, the Democratic turnout nearly doubled over 2004.