The Needle Returns for the Iowa Caucus. Here’s How It Works.

Once Iowa caucus results start coming in after 8 p.m. Eastern tonight, The Times will start publishing a live estimate of the final result, better known as the Needle.

With Donald J. Trump leading in polls by a wide margin and with much of the focus on the race for second place, our results pages will feature graphics designed to help you understand how multiple candidates are faring rather than just having a single needle displaying who is most likely to win the race.

This hypothetical chart below shows how our live estimates of the Iowa caucuses will work. Our best estimate for each candidate’s final vote share will be shown along with a range of estimates for where things might end up.

And our results pages will also include a look at an area of unusual interest — who is likeliest to take second place — as well as county-by-county maps showing where votes remain to be counted.

The fundamentals behind the Needle are surprisingly simple. In a way, it considers two big questions: Where is the vote that remains to be counted, and which candidate is faring better than expected?

As results start coming in, the Needle compares what is reported with pre-election expectations, county by county and precinct by precinct. It then estimates who will win the remaining vote based on the patterns it has seen in the results so far.

All of this, of course, is easier said than done.

The pre-election expectations are created by combining data from Times/Siena College polling, other public polls, voter registration files, the census and historical election results.

Once it receives results, the Needle uses a statistical model to spot demographic patterns that help it understand how the vote varies in different types of counties or precincts. It then blends its pre-election expectations, the output of the statistical model and the actual tabulated results into a single estimate.

Like any statistical model, the Needle benefits the more data it has, the more granular the data is, and the more detailed the data is. It fares best when an election has predictable partisan and demographic divides, which allow it to quickly judge whether a candidate is on track for victory. It benefits greatly when election officials split out returns by different methods of voting, like in-person or mail. And it benefits when it gets granular results by precinct, not just the typical results by county.

Tonight in Iowa, the Needle will have some, but not every one, of those advantages.

It will have the benefit of in-person caucuses, which avoid the challenges posed by multiple methods of voting. If all goes well, the Needle will also benefit from granular results by precinct, reported by the state’s Republican Party and collected by The Times.

But the Needle also faces one big disadvantage: It will be seeing the results of a Trump v. DeSantis v. Haley matchup for the first time. A presidential primary or caucus is not nearly as predictable as a partisan contest, when, for example, the Needle can confidently assume that New York City will be great for a Democrat.

We think the Needle is the premier tool for making sense of incomplete election results. It can offer a much clearer picture of the outcome of an election than if you just look at the early returns, which can often be biased toward a particular candidate based on which counties or precincts happen to report first.

In 2016, the Needle foresaw Mr. Trump’s surprising victory hours before the television networks. In 2020, it anticipated that Joe Biden would win Georgia even though it was days before he took the lead in the tabulated vote. In 2022, it made clear that the race for control of the House was close and that there was no red wave.

Tonight may not be so dramatic, given that Mr. Trump exceeds 50 percent in many polls. But the Needle will be learning, and telling us about important aspects of the race in real time. For example: The polls suggest that a candidate like Nikki Haley will fare relatively well in the suburbs of Des Moines — but just how well is far harder to say. The Needle may know even before the caucuses wrap up there.

The Needle does more than just create an estimate for a candidate’s final share of the vote. It also looks at a range of possibilities for how the race could go. As more votes come in, the Needle will become more certain of the final outcome.

One way the Needle conveys uncertainty is through confidence intervals, which quantify a range of reasonably plausible final outcomes. The Needle simulates many random scenarios that diverge from our best estimate at each point in the night. The extent of the randomness is informed by the model’s accuracy at comparable stages of vote counting in historical elections. The displayed confidence intervals — shown by the colored bands on the charts — contain 95 percent of those simulated results; outcomes beyond these bands aren’t impossible, but the Needle considers them unlikely.

Probabilities are worth taking seriously: A candidate with a 75 percent chance of winning, in the view of the Needle, really might win only 75 percent of the time. One in four times, that candidate will lose. Over a long primary season, there will be many times when a candidate with that chance of winning goes on to lose. That’s what we want to see: 75 percent doesn’t mean 100 percent.

In fact, looking at the Needle is never enough to say that there’s a 100 percent chance that a candidate will win. The Needle does not make race calls, as there are countless ways for our model to be led astray. In rare instances, the vote totals reported by election officials and tabulated by The Associated Press contain errors, which the Needle may assume to be factual before they are corrected. And in the past, we’ve sometimes had to pause the Needle to deal with unanticipated bugs or quirks in data feeds.

In the end, it still takes a human to figure out when a race is really over.

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