Back in February, I settled in to watch the New Hampshire primary, thinking it would be a long evening of political pundit cross talk while I waited for the results to come in. Imagine my surprise when, at 8:01, every news station called the race. I know we are living in a technological golden age, where all the information we could ever want is at our fingertips, but it was shocking to me that with 3% of precincts reporting, the news could declare who won a race.
The most effective election callers in the business are the good people at the Associated Press. The AP employs over 5,000 people to get the vote count from precincts, which they give to the national AP "vote entry clerks." Vote entry clerks will ask the "vote stringers," or the people on the ground, questions to ensure the accuracy of the results. If there's been problems at the polling site, or the results seem "suspect," the vote clerk has to know. The total number of votes entered in by the vote clerks are sent to the national office, where the AP higher-ups use them to call elections.
Before the polls even close, other members of the National Elections Pool, a network of the AP and other major news channels, will conduct exit polls. They interview a random sample of people as they leave the poll sites, and have them fill out questionnaires about their votes and opinions. I've only had the chance to vote in person a couple of times, so I'm hoping that I'll be part of an exit poll in one of New York's four primaries!
In statewide races, especially statewide races where there is one clear winner, these exit polls are often enough to call the race. Oftentimes, the AP might know who won before the polls have even closed, due to the exit polling, but politely refrains from calling the race until everyone has had a chance to vote. But that's why races sometimes get called immediately after the polls close, when few precincts are reporting.
Otherwise, races are called when the AP determines that "the trailing candidate can’t catch up, given the number of votes still outstanding and the voting history of the locations that have yet to report totals." If that sounds like weird, Nate Silver magic to you, you're right! But the AP has an accuracy rate of 99.9%, so they must be doing something right.
Wizardry aside, some elections aren't called as soon as the polls close. Sometimes, we have to wait until the AP actually inputs all the vote totals to know who won the race. This wait is sometimes as short as an hour, or in the case of Bush v. Gore, a month after the original election. Of course, in today's primaries, a month long, Bush v. Gore style recount is highly unlikely. But I'll have a lot of snacks on hand, just in case the results happen to drag on all night!