Today the polling inquiry under Pat Sturgis’ presented its initial findings on what caused the polling error. Pat himself, Jouni Kuha and Ben Lauderdale all went through their findings at a meeting at the Royal Statistical Society – the full presentation is up here. As we saw in the overnight press release the main finding was that unrepresentative samples were to blame, but today’s meeting put some meat on those bones. Just to be clear, when the team said unrepresentative samples they didn’t just mean the sampling part of the process, they meant the samples pollsters end up with as a result of their sampling AND their weighting: it’s all interconnected. With that out the way, here’s what they said.

Things that did NOT go wrong

The team started by quickly going through some areas that they have ruled out as significant contributors to the error. Any of these could, of course, have had some minor impact, but if they did it was only minor. The team investigated and dismissed postal votes, falling voter registration, overseas voters and question wording/ordering as causes of the error.

They also dismissed some issues that had been more seriously suggested – the first was differential turnout reporting (i.e, Labour people overestimating their likelihood to vote more than Conservative people), in vote validation studies the inquiry team did not found evidence to support this, suggesting if it was an issue it was too small to be important. The second was the mode effect – ultimately whether a survey was done online or by telephone made no difference to its final accuracy. This finding met with some surprise from the audience, given there were more phone polls showing Tory leads than online ones. Ben Lauderdale of the inquiry team suggested that was probably because phone polls had smaller sample sizes and hence more volatility, hence spat out more unusual results… but that the average lead in online polls and average lead in telephone polls were not that different, especially in the final polls.

On late swing the inquiry said the evidence was contradictory. Six companies had conducted re-contact survey, going back to people who had completed pre-election surveys to see how they actually voted. Some showed movement, some did not, but on average they showed a movement of only 0.6% to the Tories between the final polls and the result, so can only have made a minor contribution at most. People deliberately misreporting their voting intention to pollsters was also dismissed – as Pat Sturgis put it, if those people had told the truth after the election it would have shown up as late swing (but did not), if they had kept on lying it should have affected the exit poll, BES and BSA as well (it did not).

Unrepresentative Samples

With all those things ruled out as major contributors to the poll error the team were left with unrepresentative samples as the most viable explanation for the error. In terms of positive evidence for this they looked at the differences between the BES and BSA samples (done by probability sampling) and the pre-election polls (done by variations on quota sampling). This wasn’t a recommendation to use probability sampling (while they didn’t do recommendations, Pat did rule out any recommendation that polling switch to probability sampling wholesale, recognising that the cost and timing was wholly impractical, and that the BES & BSA had been wrong in their own way, rather than being perfect solutions).

The two probability based surveys were, however, useful as comparisons to pick up possible shortcomings in the sample – so, for example, the pre-election polls that provided precise age data for respondents all had age skews within age bands, specifically within the oldest age band there were too many people in their 60s, not enough in their 70s and 80s. The team agreed with the suggestions that samples were too politically engaged – in their investigation they looked at likelihood to vote, finding most polls had samples that were too likely to vote, and didn’t have the correct contrast between young and old turnout. They also found samples didn’t have the correct proportions of postal voters for young and old respondents. They didn’t suggest all of these errors were necessarily related to why the figures were wrong, but that they were illustrations of the samples not being properly representative – and that ultimately led to getting the election wrong.


Finally the team spent a long time going through the data on herding – that is, polls producing figures that were closer to each other than random variation suggests they should be. On the face of it the narrowing looks striking – the penultimate polls had a spread of about seven points between the poll with the biggest Tory lead and the poll with the biggest Labour lead. In the final polls the spread was just three points, from a one point Tory lead to a two point Labour lead.

Analysing the polls earlier in the campaign the spread between different was almost exactly what you would expect from a stratified sample (what the inquiry team considered the closest approximation to the politically weighted samples used by the polls). In the last fortnight the spread narrowed though, with the final polls all close together. The reason for this seems to be because of methodological change – several of the polling companies made adjustments to their methods during the campaign or for their final polls (something that has been typical at past elections, companies often add extra adjustments to their final polls). Without those changes them the polls would have been more variable….and less accurate. In other words, some pollsters did make changes in their methodology at the end of the campaign which meant the figures were clustered together, but they were open about the methods they were using and it made the figures LESS Labour, not more Labour. Pollsters may or may not, consciously or subconsciously, have been influenced in the methodological decisions they made by what other polls were showing. However, from the inquiry’s analysis we can be confident that any herding did not contribute to the polling error, quite the opposite – all those pollsters who changed methodology during the campaign were more accurate using their new methods.

For completeness, the inquiry also took everyone’s final data and weighted it using the same methods – they found a normal level of variation. They also took everyone’s raw data and applied the weighting and filtering the pollsters said they had used to see if they could recreate the same figures – the figures came out the same, suggesting there was no sharp practice going on.

So what next?

Today’s report wasn’t a huge surprise – as I wrote at the weekend, most of the analysis so far has pointed to unrepresentative samples as the root cause, and the official verdict is in line with that. In terms of the information released today there were no recommendations, it was just about the diagnosis – the inquiry will be submitting their full written report in March. It will have some recommendations on methodology – but no silver bullet – but with the diagnosis confirmed the pollsters can start working on their own solutions. Many of the companies released statements today welcoming the findings and agreeing with the cause of the error, we shall see what different ways they come up with to solve it.

The MRS/BPC Polling Inquiry under Pat Sturgis is due to release it’s initial findings at a meeting this afternoon (and a final written report in March). While I expect much more detail this afternoon they’ve press released the headline findings overnight. As I suggested here, they’ve pointed to unrepresentative samples as the main cause of the polling error back in May. There is not yet any further detail beyond “too much Labour, not enough Conservative”; we’ll have to wait until this afternoon to find out what they’ve said about the exact problems with samples and why they may have been wrong.

The inquiry conclude that other potential causes of error (such as respondents misreporting their intentions (“shy Tories”), unregistered voters, question wording and ordering) made at most a modest contribution to the error. They say the evidence on late swing was inconclusive, but that even if it did happen, it only accounted for a small proportion of the error. The inquiry also say they could not rule out any herding.

The overnight press release doesn’t hint at any conclusions or recommendations about how polls are reported or communicated to the press and the public, but again, perhaps there will be more on that this afternoon. Till then…


The monthly ICM poll for the Guardian is out today and has topline figures of CON 40%, LAB 35%, LDEM 6%, UKIP 10%, GRN 3%. The full details are on ICM’s website here and again come with some pretty candid and downbeat commentary from Martin Boon, who writes that the raw data is still heavily skewed towards Labour and that – to his mind – the existing data correction at the analysis stage isn’t succeeding in correcting it (Martin was also interviewed in Radio 4’s interesting programme this week on why the polls went wrong, as was Joe Tywman of YouGov, Damian Lyons Lowe of Survation, James Morris of GQRR and Pat Sturgis – the Chair of tomorrow’s inquiry into the polling failure).

There were three other GB voting intention polls in the weekend papers. ComRes for the Indy on Sunday had figures of CON 40%, LAB 29%, LDEM 7%, UKIP 16%, GRN 3%, Panelbase in the Sunday Times had toplines of CON 39%, LAB 31%, LDEM 6%, UKIP 14% and Survation in the Mail on Sunday had CON 37%, LAB 30%, LDEM 7%, UKIP 16%, GRN 3%.

On Tuesday the BPC/MRS’s inquiry into why the polls went wrong publishes its first findings. Here’s what you need to know in advance.

The main thing the inquiry is looking at is why the polls were wrong. There are, essentially, three broad categories of problems that could have happened. First, there could have been a late swing – the polls could actually have been perfectly accurate at the time, but people changed their minds. Secondly respondents could have given inaccurate answers – people could have said they’d vote and not done so, said they’d vote Labour, but actually voted Tory and so on. Thirdly the samples themselves could have been wrong – people responding to polls were honest and didn’t change their minds, but the pollsters were interviewing the wrong mix of people to begin with.

Some potential problems can straddle those groups. For example, polls could be wrong because of turnout, but that could be because pollsters incorrectly identified which people would vote or because polls interviewed people who are too likely to vote (or a combination of the two). You end up with the same result, but the root causes are different and the solutions would be different.

Last year the BPC held a meeting at which the pollsters gave their initial thoughts on what went wrong. I wrote about it here, and the actual presentations from the pollsters are online here. Since then YouGov have also published a report (writeup, report), the BES team have published their thoughts based on the BES data (write up, report) and last week John Curtice also published his thoughts.

The most common theme through all these reports so far is that sampling is to blame. Late swing has been dismissed as a major cause by most of those who’ve looked at the data. Respondents giving inaccurate answers doesn’t look like it will be major factor in terms of who people will vote for (it’s hard to prove anyway, unless people suddenly start be honest after the event, but what evidence there is doesn’t seem to back it up), but could potentially be a contributory factor in how well people reported if they would vote. The major factor though looks likely to be sampling – pollsters interviewing people who are too easy to reach, too interested in politics and engaged with the political process and – consequently – getting the differential turnout between young and old wrong.

Because of the very different approaches pollsters use I doubt the inquiry will be overly prescriptive in terms of recommended solutions. I doubt they’ll say pollsters should all use one method, and the solutions for online polls may not be the same as the solutions for telephone polls. Assuming the report comes down to something around the polls getting it wrong because they had samples made up of people who were too easily contactable and too politically engaged and likely to vote, I see two broad approaches to getting it right. One is to change the sampling and weighting in way that gets more unengaged people, perhaps ringing people back more in phone polls, or putting some measure of political attention or engagement in sampling and weighting schemes. The other is to use post-collection filters, weights or models to get to a more realistic pattern of turnout. We shall see what the inquiry comes up with as the cause, and how far they go in recommending specific solutions.

While the central plank of the inquiry will presumably what went wrong, there were other tasks within the inquiry’s terms of reference. They were also asked to look at the issue of “herding” – that is, pollsters artificially producing figures that are too close to one another. Too some degree a certain amount of convergence is natural in the run up to an election given that some of the causes of the difference between pollsters are different ways treating things like don’t knows. As the public make their minds up, these will cause less of a difference (e.g. if one difference between two pollsters is how they deal with don’t knows, it will make more of a difference when 20% of people say don’t know than when 10% do). I think there may also be a certain sort of ratchet effect – pollsters are only human, and perhaps we scrutinise our methods more if we’re showing something different from anyone else. The question for the inquiry is if there was anything more than that? Any deliberate fingers on the scales to make their polls match?

Finally the inquiry have been asked about how polls were communicated to the commentariat and the public, what sort of information is provided and guidance given to how they should be understood and reported. Depending on what the inquiry find and recommend, in terms of how polls are released and reported in the future this area could actually be quite important. Again, we shall see what they come up with.

Survation had a new poll of Scottish voting intentions in the Holyrood election this week. As usual in the present Scottish political scene they show a towering SNP lead, with Labour second and the Conservatives in third. Constituency voting intentions are SNP 52%, LAB 21%, CON 16%, LDEM 7%; Regional list intentions are SNP 42%, LAB 20%, CON 16%, GRN 9%, LDEM 8%, UKIP 5%. Tabs are here.

Meanwhile the weekly ICM EU referendum tracker has started up again after the Christmas break. Their final poll of 2015 had been an unusual 50-50 split but the latest poll has reverted to the norm – REMAIN 44%, LEAVE 38% (the equivalent, after don’t knows are excluded) of REMAIN 54%, LEAVE 46%. Full tabs are here.