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How the Iowa caucuses work

In just four days, people from across the state of Iowa will head to their local community centers, gymnasiums and churches to kick off the process of picking the next president of the United States.

Here's a primer on what to expect:

What is a caucus?

The Democratic Party of Iowa (IDP) calls it an "organizational gathering of neighbors...where Democrats meet to conduct party business and declare their Presidential preference." The Republican Party of Iowa says it's "a meeting of a group of persons in the same political party who select candidates or decide policy." Registered Republicans and Democrats gather to express their choice of candidate for president, which will determine how their state's delegates vote at the national conventions that summer.

Where do they take place?

Community centers, school gymnasiums, churches -- sites that offer space for a number of people to gather. There are 1,681 precincts that will hold caucuses - about 1,100 spots for Democrats and 900 for Republicans. The Democrats will also hold a "tele-caucus" for people who are living overseas and several "satellite caucuses" at places like nursing homes, or large businesses that have a number of employees working an overnight shift, to expand access.

When do they start?

Caucuses are Monday, February 1, starting at 7 p.m. CST. There is no set end time.

What happens in an actual caucus?

For Republicans, it's a fairly straightforward process. Caucus attendees show up, and they can hear messages of support from either one of the candidates, a representative from the campaign, or a supporter. People cast a ballot for their preferred candidate, and national convention delegates are awarded proportionally based on the results.

The Democrats' caucus is more complicated. Candidates are competing to win over a set number of county convention delegates, which are determined by the Democratic turnout during the most recent presidential and gubernatorial elections. That number translates into a "state delegate equivalent," based on a ratio of state to county convention delegates. When you hear about which Democrat "won" the Iowa caucus, you're really talking about who got the highest number of state delegate equivalents. (I mentioned this was complicated.)

First, voters show up to their precinct site. Then they'll divide into presidential preference groups for the candidates they are supporting. For a candidate to be awarded any delegates out of that precinct, they'll need to be "viable" -- that is, they must have the support of at least 15 percent (or, in some cases, more) of the people in attendance.

If a candidate is not viable, their supporters can try to win over other caucus goers to meet the required threshold. Or they can disband and support the viable candidates. Their other option is to remain uncommitted entirely.

Based on the final results of the preference vote, each candidate will receive a proportional number of the county convention delegates, and "state delegate equivalents." The exact delegate selection continues at the county and state conventions later in 2016, but generally reflects the presidential preference vote.

This year, results from both Democratic and Republican precincts will be reported by a new mobile app the parties created through a partnership with Microsoft and Interknowlogy. The Democratic Party said the app "will allow the party to quickly flag any discrepancies, and then allow the IDP to provide secure, accurate, and timely results to both the public and members of the media."

How many people will participate?

In 2012, 121,503 Republicans showed up to caucus -- just shy of 20 percent of Republicans who were registered to vote at the time. The last contested Democratic caucus, in 2008, drew 239,872 people, or nearly 40 percent of registered Democrats -- a huge turnout number attributed to the excitement about Barack Obama's candidacy. In a more typical year, 2004, only 124,331 Democrats, or 23 percent of those registered to vote, showed up.

It's impossible to say exactly how many will show up this year, but Iowa State University Political Science Professor Steffen Schmidt predicted this caucus will be "one of the biggest ever" and possibly larger than the historic 2012 turnout.

"The stakes are incredibly high and number two both parties have absolutely electrifying candidates to offer," he told CBS News.

But the current weather forecast predicts a chance of snow on Tuesday night in parts of Iowa, which could keep people at home.

How will turnout affect the results?

High turnout will benefit the two candidates who are most likely to draw first-time caucus-goers or independents - Vermont Sen. Bernie Sanders, a Democratic candidate, and businessman Donald Trump, a Republican candidate.

In an interview with NBC News Wednesday, Sanders said, "On caucus night in Iowa you will be able to tell very early, I think, who wins and who loses. If there's a large voter turnout, we're going to win."

One polling organization, Monmouth University, is predicting that 170,000 people will show up to caucus on Monday - nearly 50,000 more Republicans than the total in 2012. According to their projections, increasing turnout to 200,000 voters would grow Trump's lead over next-closest rival Sen. Ted Cruz from 7 percentage points to 11 percentage points. But if turnout is 130,000 -- closer to the historical average -- Trump and Cruz could end up in a tie.

Why is it done this way?

It's a somewhat arcane system, sure. But every state has written different rules for how they carry out the primary or caucus process. And Iowa has been doing this for years.

"It stayed this way for a while for tradition, I think, but after that, Iowa is also a proving ground," Norm Sterzenbach, the former executive director of the Iowa Democratic Party, told CBS News. "Campaigns can come here on the Democratic side and get ready for the battles down the road."

"We're basically requiring campaigns to prove themselves and they're organizing ability by competing in Iowa," he said.

Schmidt also said Iowans have proven their good judgment at this point.

"Since 1792 no one who has come in lower than third in Iowa has gotten the nomination, and that's because probably Iowans correctly point at the three best candidates and then give advice to the other states and say 'here's what we think' and the other states all the way to the nomination for the most part say, 'Yeah those look pretty good," he said.

  • Rebecca Kaplan

    Rebecca Kaplan is a political reporter for CBSNews.com.