If you've ever tried to follow a U.S. election's results live, you've most likely heard or seen races being called while simultaneously hearing or seeing a tally of "1 percent reporting" in the state that was just called. This, as well as how quickly some states occasionally declare their winners, can be baffling, so how is a presidential election called? Here are a few of the methods used at polling stations and news organizations to call an election winner.
One of the major factors available to those who count the results are early votes. This year's early voting numbers are record-breaking, with 49 million people who have already submitted their ballots. This gives voting stations a good estimation of where the state is headed. Early voting has seen a surge in swing states in particular — almost half of Florida's registered electorate voted early, something that will certainly help in calling the most divided states in the country.
Major news networks also rely on data from Election Day exit polls. Exit polling is conducted on a county-by-county basis, and the data collected from different voting stations allow networks to collect accurate estimates of which counties in a certain state have voted Republican or Democrat. While these polls don't provide the hard data of actual votes numbers, they frequently give accurate enough estimations to allow networks to call a winner. In addition to exit polls, networks also make use of absentee polls.
As for the odd frequency of results being announced despite a low percentage of precincts being listed as actively reporting, the Associated Press gives the following explanation:
Frequently the transmission of precinct results to the county or city is manual — someone actually bringing a thumb drive from an optical scanner to the county, or bringing the precincts’ actual ballots to the county to be scanned there. It is not unusual for there to be mislaid cartridges or ballots left in a locked-up polling place on election night.
Even when the transmission of the precinct results to the county is electronic, there are inevitably precincts that don’t get sent because of bad thumb drives, no Wi-Fi connection, etc. And in some primaries in smaller counties there might literally be no voters casting a ballot for one party. So that might cause confusion as to whether that precinct has reported.
Close races tend to take longer to call, due to the need for recounts. With the current election having gotten closer in the past week, there's the possibility of a longer wait than the one we went through in 2012, but some are already anticipating an early result.