Months after opinion polls failed to predict Britain's vote to leave the European Union, their reliability was called into question once again after they backed the wrong horse in the US presidential election.
Most opinion polls in the run-up to Tuesday's election had put Democrat Hillary Clinton firmly ahead of US President-elect Donald Trump.
An average of national opinion surveys gave her a two percentage point advantage on Monday, the day before the vote, and the liberal New York Times gave her an 84-per-cent chance of winning.
Bookmakers also backed Clinton, with Ireland's Paddy Power even paying out 1 million euros ($A1.4 million) three weeks before the vote to punters who had bet on Clinton.
The fiasco appeared bound to draw comparisons with Britain's Brexit vote in June, before which opinion polls - and bookmakers - also failed to forecast Brexit.
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Trump himself, who has dismissed opinion polls as "crooked," appeared to indicate that he thought he would lose by repeatedly asserting that the election was "rigged" against him at his final rallies.
He tweeted on Tuesday that "Utah officials report voting machine problems across entire country."
He also repeatedly refused to say whether he would concede the vote to Clinton if she won. "We'll see what happens," he told reporters after casting his vote in New York on Tuesday.
His tone reflected that of Nigel Farage, Britain's chief campaigner to leave the EU, who before results were in on the night of the June 23 vote conceded that it "looks like Remain will edge it."
British polling expert John Curtice had said too much attention was focused on nationwide polls, rather than the accuracy of individual state polls.
"The winner in a state takes all of its electoral college votes irrespective of the narrowness of their victory," he wrote in a column for British newspaper The Independent.
In the end, Trump triumphed in key swing states including Florida Pennsylvania and Ohio, sending Clinton supporters into shock and world stock markets downward.
As Trump himself put it at a rally in North Carolina the day before the election, "It's going to be Brexit plus, plus, plus."
