It's the day after Election Day, and most races around the country have been called.

But in Arizona, a few high-profile contests are too close to call. And because of the way the state's largest counties report their results, we may not know who has won for awhile.

The Arizona Republic estimated Wednesday that 650,000 ballots are left to be counted. Here's how we came to that number and why the number of "precincts reporting" doesn't tell the whole story.

How did The Republic arrive at its estimate?

The Republic incorporated historical turnout numbers, early voting and Election Day returns to estimate total turnout for this year's midterm elections. Those numbers are updated every time a county or the state releases an estimate of how many votes are left to be counted. There were an estimated 650,000 votes outstanding at noon Wednesday.

Why does the Secretary of State say most precincts have reported?

The precincts reporting number only refers to the number of precincts reporting Election Day votes. Early votes, which typically make up three-fourths of all ballots cast, and provisional ballots aren't included in that number.

With so many ballots to be counted, how can races be called?

Races are called based on what share of the remaining votes a trailing candidate would have to win to change the outcome. As more votes are counted, a trailing candidates must take a higher and higher shares of the remaining votes to win. At a certain point, it becomes unlikely that one candidate will receive that share of remaining votes, so a race can be called for the leading candidate.

In some races, such as the 2018 governor's race, candidates have such large early leads based on early results that it becomes highly unlikely the trailing candidate can make up the difference.

MORE: The McSally-Sinema Senate race is too close to call. Now what happens?

How does this affect the U.S. Senate and state superintendent races?

In very close races, it's impossible to know with certainty who will win until enough votes are tallied. Voters should have a clearer idea about the likely final result of most close races by Friday. Some races, including the closely watched Senate race between Kyrsten Sinema and Martha McSally, could take even longer.