Donald Trump has been citing Brexit as the model of how he could win the election despite expections, his surrogates of how there might be a shy Trump vote, like Brexit. So what, if any, lessons can we learn about the US election from recent polling experience in Britain?

In 2015 the British polls got the general election wrong. Every company had Labour and Conservative pretty much neck-and-neck, when in reality the Conservatives won by seven points. In contrast, the opinion polls as a whole were not wrong on Brexit, or at least, they were not all that wrong. Throughout the referendum campaign polls conducted by telephone generally showed Remain ahead, but polls conducted online generally showed a very tight race. Most of the online polls towards the end of the campaign showed Leave ahead, and polls by TNS and Opinium showed Leave ahead in their final eve-of-referendum polls.

That’s the first point that the parallel falls down – Brexit wasn’t a surprise because the polls were wrong. The polls were showing a race that was neck-and-neck. It was a surprise because people hadn’t believed or paid attention to that polling evidence. The media expected Remain would win, took polls showing Remain ahead more seriously and a false narrative built up that the telephone polls were more accurately reflecting the race when in the event, those online polls showing leave ahead were right. This is not the case in the US – the media don’t think Trump will lose because they are downplaying inconvenient polling evidence, they think Trump will lose because of the polling evidence consistently shows that.

In the 2015 general election however the British polls really were wrong, and while some of the polls got Brexit right, some did indeed show solid Leave victories. Do either of those have any relevance for Trump?

The first claim is the case of shy voters. Much as 1948 is the famous examples of polling failure in the US, in this country 1992 was the famous mistake, and was put down to “Shy Tories”. That is, people who intended to vote Conservative, but were unwilling to admit it to pollsters. Shy voters are extremely difficult to diagnose. If people lie to pollsters about how they’ll vote before the election but tell the truth afterwards, then it is impossible to distinguish “shy voters” from people changing their minds (in the case of recent British polls, this does not appear to be the case. In both the 2015 election and the 2016 EU referendum recontact surveys found no significant movement towards the Conservatives or towards Leave). Alternatively, if people are consistent in lying to pollsters about their intentions beforehand and lying about how they voted afterwards, it’s impossible to catch them out.

The one indirect way of diagnosing shy voters is to compare the answers given to surveys using live interviewers, and surveys conducted online (or in the US, using robocalls – something that isn’t regularly done in the UK). If people are reluctant to admit to voting a certain way, they should be less embarrassed when it isn’t an actual human being doing the interviewing. In the UK the inquiry used this approach to rule out “shy Tories” as a cause of the 2015 polling error (online polls did not have a higher level of Tory support than phone polls).

In the US election there does appear to be some prima facie evidence of “Shy Trumpers”* – online polls and robopolls have tended to produce better figures for Donald Trump than polls conducted by a human interviewer. However, when this same difference was evident during the primary season the polls without a live interviewer were not consistently more accurate (and besides, even polls conducted without a human interviewer still have Clinton reliably ahead).

The more interesting issue is sample error. It is wrong to read directly across from Brexit to Trump – while there are superficial similarities, these are different countries, very different sorts of elections, in different party systems and traditions. There will be many different drivers of support. To my mind the interesting similarity though is the demographics – the type of people who vote for Trump and voted for Brexit.

Going back to the British general election of 2015, the inquiry afterwards identified sampling error as the cause of the polling error: the sort of people who were able to be contacted by phone and agreed to take part, and the sort of people who joined online panels were unrepresentative in a way that weights and quotas were not then correcting. While the inquiry didn’t specify how the samples were wrong, my own view (and one that is shared by some other pollsters) is that the root cause was that polling samples were too engaged, too political, too educated. We disproportionately got politically-aware graduates, the sort of people who follow politics in the media and understand what is going on. We don’t get enough of the poorly educated who pay little attention to politics. Since then several British companies have adopted extra weights and quotas by education level and level of interest in politics.

The relevance for Brexit polling is that there was a strong correlation between educational qualification and how people voted. Even within age cohorts, graduates were more likely to vote to Remain, people with few or no educational qualifications were more likely to vote to Leave. People with a low level of interest in politics were also more likely to vote to Leave. These continuing sampling issues may well have contributed to some of those pollsters who did it wrong in June.

One thing that Brexit does have in common with Trump is those demographics. Trump’s support is much greater among those without a college degree. I suspect if you asked you’d find it was greater among those people who don’t normally pay much attention to politics. In the UK those are groups who we’ve had difficulty in properly representing in polling samples – if US pollsters have similar issues, then there is a potential source for error. College degree seems to be a relatively standard demographic in US polling, so I assume that is correct already. How much interest people have in politics is more nebulous, less easy to measure or control.

In Britain the root cause of polling mishaps in 2015 (and for some, but not all, companies in 2016) seems to be that the declining pool of people still willing to take part in polls under-represented certain groups, and that those groups were less likely to vote for Labour, more likely to vote for Brexit. If (and it’s a huge if – I am only reporting the British experience, not passing judgement on American polls) the sort of people who American pollsters struggle to reach in these days of declining response rates are more likely to vote for Trump, then they may experience similar problems.

Those thinking that the sort of error that affected British polls could happen in the US are indeed correct… but could happen is not the same as is happening. Saying something is possible is a long way from there being any evidence that is actually is happening. Some of the British polls got Brexit wrong, and Trump is a little bit Brexity, therefore the polls are wrong really doesn’t carry water.

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*This has no place in a sensible article about polling methodology, but I feel I should point out to US readers that in British schoolboy slang when I was a kid – and possibly still today – to Trump is to fart. “Shy Trump” sounds like it should refer to surreptitiously breaking wind and denying it.


451 Responses to “What can British polling mishaps tell us about the US election?”

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  1. post #2 – Brexiters’ expectations
    @Carfrew 27/10 23:00
    quoting Mathew Smith in ‘The Times’

    ‘Of all the negative side effects of Brexit, the one Leave voters most expect is that the price of their weekly shop will go up. However, this still only applies to just a quarter of Leave voters, fewer than half the 57 per cent who think Brexit will make no difference to the price of groceries.’

    ‘And herein lies one of the problems for those seeking to keep Britain in the EU: Leave voters essentially see Brexit as a no-risk proposition. ‘

    Well, at some point, reality bites. But not yet – people are so inured to petrol prices moving up and down that they will neither know nor care about the causes.

    When uncompensated inflation starts affecting people’s grocery basket, it would be a mistake to assume that people will make the link to Brexit. It is, instead, a testable proposition that could be polled.

    Taking the Marmite ‘debate’ into consideration, I would suggest a three-way choice (their order unfortunately may affect the answers), as follows:

    Do you consider that recent price rises are mainly or mostly caused by:
    1) Brexit
    2) Unfair prices by international companies
    3) The EU attacking British consumers

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