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Why was the South Carolina primary called for Biden so early?

The early forecast call by the networks for Joe Biden seemed to catch people off-guard, but it's not out of the ordinary for an early projection.

COLUMBIA, S.C. — Former Vice President Joe Biden was declared the projected winner of the South Carolina primary mere moments after polls closed, before even one vote was counted. But how could that be possible? 

[South Carolina Primary Election Results: Numbers live ]

The Associated Press, CBS, and all the other major networks called it by 7:01 p.m. Eastern Time. Polls closed statewide one minute earlier. But that speed had many people online asking how that could be so.

In short, networks do surveys of voters as they leave the polls which are commonly called exit polls. Networks have been using them for over two decades and they have a high degree of accuracy, particularly for races that aren't close contests.

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And that appears to be the case Saturday. 

CBS, for example, released some of that data about an hour before the polls closed, and it showed some decisive data that pointed strongly in favor of Biden.

In the exit poll, almost half of primary voters said South Carolina U.S. Rep. Jim Clyburn's endorsement of Biden this week played an important factor in their vote. Clyburn is a civil rights veteran who's been in state and local politics for five decades and is one of the most powerful Democrats in Washington. 

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Most respondents (51 percent) also said they wanted to return to former President Barack Obama's policies. A much smaller number, 29 percent, said they wanted there to be more liberal policies. Sanders, a self-described Democratic-socialist, is considered far more ideological liberal than Biden.

Combine all those data points together, plus some numbers that the networks keep to themselves, and it starts to paint a picture with why they didn't wait long. 

The Associated Press has a slightly different system, which it calls AP Vote Cast. 

AP Vote Cast debuted in the 2018 midterm elections., and in the AP's own words, is "designed to tell the Election Day story of why the winners won." it developed the system over the previous decade, and interviews thousands of voters. It uses a combination of mail in surveys and phone and online polls. 

The AP says in 2018, it correctly predicted 92 percent of the races. 

While Saturday may have seemed shocking due to the high-profile nature of the contest, race calls at polls close time are very common. Many incumbents easily win, especially if they are in deep-red or deep-blue parts of the country. 

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