How accurate are exit polls?

A key moment on election night will be the first survey of voter data

After months of rallies, interviews, speeches and town hall events, Donald Trump and Kamala Harris will be anxiously following every development on election night.
One key moment will take place at 5pm ET, when the networks release the early results of the exit poll.
The survey will not provide an instant answer on the election winner, and its numbers will be tweaked throughout the night as more data from polling places across the US comes in.
But it could provide early indications on which campaign has managed to turn out their supporters to the polls, and who is falling short or over-performing with key voter groups.
An exit poll is a survey that establishes how voters have voted in an election and their reasons for doing so.
For a presidential election, the exit poll has three parts that take account of the various ways that voters may choose to cast their ballot.
Those who vote in-person in the weeks leading up to the election – known as “early voting” – are sampled at hundreds of polling locations across the country, as are those who do so on election day.
The final part of the poll targets voters who vote early or by mail, who are contacted by phone call, email, or text.
Voters are typically asked 20 questions on who they have voted for, demographic factors including age, gender, and race, and the reasons for the way they voted.
News organisations can start reporting results from the exit poll after the embargo period ends at 5pm ET on election day.
It can only forecast a winner in a race if all the polls have closed in the state, and even then it has to be a state where the margins of victory are sufficiently large.
Election-watchers should be sceptical of the first exit poll results when they are released at 5pm ET.
These results will be adjusted several times over the course of the night because polling locations are still open. Those who have been surveyed earlier in the day tend not to be representative of the wider electorate.
For instance, those who cast their ballots earlier in the day tend to be older than the average voter. Republicans have an edge among those aged over 50 years old.
As the night goes on and more voters are added to the sample, the exit polls move closer to the final result.
Even then, the polls go through modelling that can dramatically alter their final conclusions.
The 2016 exit poll made headlines when it suggested that Donald Trump had won the support of the majority of white women.
In fact, when reviewed by the Pew Research Centre, it was 47 per cent of white women who supported Trump – cutting the initial figure by six percentage points.