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How YouGov correctly predicted the UK election where other pollsters failed

Tom Macaulay | July 13, 2017
The polling firm YouGov correctly predicted the UK election thanks to an experimental statistical model called MRP.


Against the odds?

The loss of Canterbury to Labour after 99 years in Tory hands was a shock to most, but YouGov saw it coming. Its prediction was primarily based on Canterbury having a large presence of remain voters and students.

"That was just what the data says," says Rivers. "The thing that you have to understand about this was it was taking eight hours on a forty core system at AWS to estimate the model, so we weren't going through and adjusting predictions at any place or doing anything special. It was the overall model. That was a constituency where the remain vote was a helpful predictor of what was going to happen this year."

The other pollsters likely overcompensated for their previous failure to predict a Tory majority in 2015 by overcorrecting previous errors manipulating their data to fit their beliefs

Ipsos Mori for example, made a last minute adjustment to their neck-and-neck predictions based on turnout to transform it into a comfortable Conservative win.

"They just believed in their heart it was going to come out this way and they tortured their data to make that happen," says Rivers. "So I think one of the lessons is just to listen to your data."


Hits and misses

YouGov didn't get everything right, including some surprises in Scotland where they overrated the SNP's chances. The Conservative vote share lead was also smaller than expected, but they still won slightly more seats than the poll predicted.

"The Conservatives won, in the end, a lot of close races," says Rivers. "The Amber Rudd seat was one where we actually predicted she would lose by a small amount.

"She won by a small amount, so we felt the quality of that prediction was pretty good. When it popped up people raised their eyebrows and said you're not really predicting the home secretary's going to lose. The test is does it tell you the race is close when you have a close race. You're going to be lucky in some places like we were in Canterbury and slightly unlucky in the Amber Rudd seat."

YouGov's MRP model had previously successfully predicted the EU referendum result, but it narrowly failed to predict Trump's victory in the 2016 US Election. YouGov has less experience operating in the US market, and covering its less densely populated area presented different challenges.

They correctly predicted that Hillary Clinton would narrowly win the popular vote, but were mistaken in forecasting that she would also edge the Electoral College. This was largely because the key Midwestern battleground states were too close to call, and a higher turnout of Trump supporters shifted some of them his way.


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