Donald Trump’s election victory came as a shock to many, proving wrong the many reporters, pollsters and political experts who had tipped Hillary Clinton for the White House.
But Allan Lichtman, a political analyst who has correctly predicted the results of every presidential election since 1984, correctly foresaw that Mr Trump would be the 45th US President.
Professor Lichtman, a historian at American University, said the Clinton-Trump contest had been the hardest race he had ever attempted to call.
But he calculated the outcome using a rigid method that has never failed him in nearly 30 years, saying the result was, in part, because Ms Clinton lacked the "charisma" of Barack Obama.
Unlike many experts who fixated on Mr Trump's controversial campaign when assessing the election outcome, Professor Lichtman's calculations largely focused on the incumbent party's potential for another victory.
While he described Mr Trump as the first US presidential candidate to have “spent his life enriching himself at the expense of others" and described him as “a serial fabricator, making up things as he goes along”, he also looked closely at Ms Clinton when assessing the outcome.
Making his prediction in September, Professor Lichtman said: “One of my keys is whether or not the sitting president is running for re-election, and right away, [the Democrats] are down that key.
“Another one of my keys is whether or not the candidate of the White House party is, like Obama was in 2008, charismatic. Hillary Clinton doesn't fit the bill.”
He added: “Donald Trump has made this the most difficult election to assess since 1984. We have never before seen a candidate like him, and Donald Trump may well break patterns of history that have held since 1860.
“But based on the 13 keys, it would predict a Donald Trump victory.”
The 13 keys refer to a method Mr Lichtman has used to predict the outcomes of all eight American presidential elections from 1984 to 2012. The system entails “mathematically and specifically” measuring the performance of the party in office.
“It is a historically based prediction system. I derived the system by looking at every American presidential election from 1860 to 1980," Professor Lichtman has said.
"The keys are 13 true/false questions, where an answer of 'true' always favours the re-election of the party holding the White House, in this case the Democrats. And the keys are phrased to reflect the basic theory that elections are primarily judgments on the performance of the party holding the White House."
Professor Lichtman said he came across the method on meeting a Russian geologists who wanted to collaborate using the methods of earthquake prediction to predict elections.
