Shortly before 9 p.m. Eastern on Monday, we expect the first returns from the Iowa Democratic caucuses. If you choose to follow the results live tonight, we hope you’ll do it with us: We’ll have results, analysis from our reporters and interactive maps.

We’ll also have what we call a live forecast, known to many readers simply as “The Needle.” Below, we’ve provided answers to some common questions about the needle and our plans to use it during the caucuses.

What is the needle?

It’s the visual representation of The Times’s live election night forecast. It combines initial election returns with other sources of data to estimate the final result of an election before all the votes have been counted.

Before the election, we use polling data, census data and prior election results to come up with a baseline estimate for how every precinct or county will vote. As the results come in, we compare the actual returns with our baseline. We then adjust our expectations for the vote that’s left: If a candidate is running ahead of our pre-election expectations in the areas that are counted, we might assume that he or she is also poised to do better in the demographically similar areas that remain.

Why do we do it?

The needle gives many readers the piece of information they want more than anything else on election night: It tells them who is on track to win the election. And when the needle doesn’t know, it tells them that, too.

The needle works because incomplete election results are often deeply unrepresentative. In some races — including the Iowa caucuses — predominantly rural places tend to be the first ones to report votes. The raw vote count can be misleading evidence about who is actually in a better position to win, even after most votes have been reported.

Sophisticated election analysts and decision desks know this. Most casual followers of an election do not. The needle is a shortcut to most of the things we know about political geography.

In 2016, the needle helped us realize fairly early on election night that Donald J. Trump was on track to win. We showed him with an edge in Pennsylvania, even when Hillary Clinton had a double-digit lead in tallied votes.

Using similar methods in 2014, we were able to tell readers that Mark Warner, a Virginia Democrat, was probably on his way back to the Senate despite trailing all the way through 99 percent of precinct reports. In Alabama’s special Senate election in 2017, the needle showed that the Democrat, Doug Jones, had a path to victory before many analysts did.

For those who wonder whether the world really needs the election needle, we realize the actual results will emerge soon enough. But we also think that the millions of people who follow election night results online ought to have the context to understand them as well as experts do.

Is it harder in Iowa?

The Iowa caucuses pose a number of challenges for election night forecasting.

First, Iowa is the first contest of the Democratic primary season. We think we have a pretty good idea of where each candidate should fare well, based on polling data and past elections. But Joe Biden and Bernie Sanders have never faced off before; it’s hard to be absolutely sure who would have the edge in rural areas, for instance, until we start to see some votes. Certainly, we’re far less confident of who will win a given county in Iowa than we would be in a general election between Democrats and Republicans.

Second, Iowa polls have often been inaccurate, and it is hard to know how changes in the race — like a hypothetical election night surge from Amy Klobuchar — might wind up upsetting our expectations. The large number of candidates adds volatility to our expectations.

Third, the caucuses are complex and involve multiple stages of voting. It is hard to know how each of these stages will play out over all, or by precinct. Voters can make strategic decisions in a specific caucus site that we do not anticipate.

A final complicating factor is that in 2020, the Iowa caucuses aren’t just in Iowa. There will be satellite caucuses in retirement communities, mosques and people’s homes, some as far away as Paris, Scotland and Tbilisi, Georgia. Without any prior voting history there, or knowledge of the demographics of who will attend, it’s hard for us to know what to expect from these locations.

Nonetheless, we do have some things going for us:

• In general, the demographic divisions in the Democratic field are good news for the needle. Based on our polling, we have a decent idea of the likely geography of the Iowa caucuses: Bernie Sanders has an edge in many urban areas and college towns; Pete Buttigieg is relatively strong in rural areas; Elizabeth Warren does well in areas with more college graduates; and the Des Moines suburbs are a battleground.

• We expect to receive results by precinct, rather than by county; these more granular results will help us make sense of the data fairly quickly.

• Historically, there does not appear to be a big difference between the preference of voters when they show up to caucus and the actual state delegate equivalency results, based on entrance poll data.

• The complicated nature of the delegate math is easy for computers, but hard for humans. Even an expert in Iowa’s political geography might have a hard time translating first alignment votes to delegate totals for thousands of precincts in real time. Advantage, Needle.

Can the needle be ‘wrong’?

Yes, depending on what you mean by “wrong.”

Once all of the votes have been counted, the needle will — by definition — match up perfectly with the final result.

But before all votes have been counted, the needle may suggest that one candidate has a pretty good chance of winning, only to have someone else eventually win. A 65 percent chance of winning means losing quite often. Likely to happen does not mean certainly will happen. We don’t quite think of this as “wrong.” (We know that some people do. This year, the needle will also describe probabilities with a simple phrase; it would describe the odds of someone with a 65 percent chance of winning as “likelier than not.”)

The needle can also be wrong by reacting too quickly to unrepresentative returns, or by failing to anticipate them.

If, by random chance, a candidate’s very best or worst precincts are the first ones to report, the needle may be misleading. That problem briefly confused the needle in the 2017 Alabama Senate race between Mr. Jones and Roy Moore.

For a few minutes, the needle projected Republican turnout to be abysmal — but this was because the first counties to complete their count included some of the weakest Republican turnout of the night. The magnitude of our error at that point was huge — nearly seven percentage points. But our model quickly realized that Republican turnout was going to be higher in other counties.

On the other hand, if the results are reported in a perfectly representative way, the needle may not offer much insight beyond raw vote totals.

How many needles will there be for Iowa?

Our main forecast will be for state delegate equivalents, a measure reflecting a precinct’s vote at final alignment (after caucusgoers whose initial candidate didn’t meet a minimum reallocate themselves). The candidate with the most state delegate equivalents has traditionally been declared the winner of the Iowa caucuses, and in 2020 The New York Times, along with most news media organizations, will again characterize the candidate with the most state delegate equivalents as the winner.

But for the first time, the Iowa Democratic Party is making data available from other steps of the precinct caucuses. We’ll offer live estimates on those measures, too:

• The first alignment reflects the first preference of caucusgoers when they arrive at their precinct.

• The final alignment reflects the preference of caucusgoers after the supporters of nonviable candidates have an opportunity to realign with candidates who remain viable in their precinct. Usually, a candidate needs 15 percent in a precinct to be viable.

• The pledged delegate count is awarded based on the number of state delegate equivalents won by each candidate statewide and by congressional district.

Believe it or not, it is possible for different candidates to win each measure.

We’ll need precinct data (as opposed to county data) to make reliable forecasts for all of these measures. If we don’t get this data, there will be not be a needle. (In 2018, data problems caused us to take the needle offline in the middle of the special election of Pennsylvania’s 18th district, ultimately won by Conor Lamb, a Democrat.)

Does the needle ‘call’ a race?

No. Calling is for humans.

The needle reflects an estimate of the size and the makeup of the outstanding vote. And it may fail to anticipate returns that don’t fit the patterns we’ve seen until a certain point.

For this reason and others, the needle is not used to make definitive statements about who has won.