How and why AP called Clinton-Sanders race when it did

WASHINGTON -- The question burned late into the night: Who won the Iowa Democratic presidential caucuses, Hillary Clinton or Bernie Sanders? There would be no answer until well into the next day.

News organizations widely rely on The Associated Press to call the winner in elections and, in presidential caucuses and primaries, to determine the number of delegates won by candidates to the nominating conventions.

On Monday night, a close approximation of the delegates won by Clinton and Sanders was known. But the winner wasn't -- not until early Tuesday afternoon.

Questions and answers about how the AP determined Clinton won Iowa and how it allocated delegates in the Democratic contest:

Q: Why didn't the AP call the winner in the Iowa Democratic caucuses on Monday night?

A: The race was too close. Clinton's margin over Sanders was so minuscule, it risked being erased if vote-counting errors were discovered later.

The AP uses data from three sources to call races. First, entrance or exit polls of voters arriving to caucuses or leaving voting precincts (on Monday, entrance polls were used). Second, an analysis of actual votes from a random sample of precincts across each state. Third, the AP vote count, which tabulates all actual votes as they are reported from precincts to counties, or towns or parishes.

For the closest races, the AP depends heavily on actual vote returns as they slowly or rapidly flow in, depending on circumstances, to make a call.

Q: What changed Tuesday afternoon that led the AP to call the race for Clinton?

A: Three factors enabled the AP to call the race for Clinton just after noon: All the results had been tallied, the Iowa Democratic Party told the AP it would not conduct a recount of any results, and the Sanders campaign said it was not interested in challenging any results.

Clinton won in the end by less than three-tenths of 1 percent.