Why are the media reporting different US election results?

As the US election slowly draws to its conclusion, one of the areas of confusion is that different media outlets are showing different results for the electoral votes.

The president is elected by winning at least 270 electoral college votes, not the outcome of the popular vote. Because there is no centralised federal election system, it has become tradition in the US that “decision desks” at media organisations make a call that states have been won by one candidate or the other when enough votes have been counted. States that are too close to call – such as Nevada and Georgia at the moment – remain in the balance until a network “calls” them.

The Guardian uses the data collected and analysed by the Associated Press (AP) news agency as the source for when we will call election results. There are a number of other highly reputable election decision desks in US media, including NBC, Fox News and others. They may call races earlier or later than AP. US networks obviously use the decisions from their own desks – other channels may chose to follow one, or wait for two desks to call a state before they count it.

This year, Arizona has brought this into sharp relief. Our current total of 264 electoral votes for Joe Biden includes the fact that AP has called Arizona for the Democratic nominee. Not all decision desks have yet.

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AP has issued this guide to all of the states it has called. This is what it says about Arizona:

The AP called the race at 2.50am EST Wednesday, after an analysis of ballots cast statewide concluded there were not enough outstanding to allow Trump to catch up. With 80% of the expected vote counted, Biden was ahead by 5 percentage points, with a roughly 130,000-vote lead over Trump, with about 2.6m ballots counted. The remaining ballots left to be counted, including mail-in votes in Maricopa county, where Biden performed strongly, were not enough for Trump to catch up to the former vice-president.

The Trump campaign disagrees. Biden’s lead is currently down to about 68,000 votes in the state – or less than three percentage points – with 88% of the votes counted.