Major polls forecasted a hung Parliament, underestimating the Tory vote by an average of 4.2 points and overestimating the Labour vote by an average of 2.4. Pollsters changed practices viz. British Polling Council recommendations. Is there now a consensus on :

  • Why polls were wrong in 2015?
  • What they need to do differently to make more accurate predictions for the 2017 election onwards?
  • Is there an example poll to look at? Polls always predict a range of outcomes, so to even begin to answer the margin of error (and probably a few other things) would need to be known. Commented Apr 24, 2017 at 13:48

3 Answers 3


What they need to do differently to make more accurate predictions for the 2017 election onwards?

A few fault points in polling.

  1. Sampling: most polls used voter rolls and telephones (land line) to reach its samples. It is unlikely to represent the actual voters.

  2. Reweighing: as the samples don't quite represent the actual voters, pollsters reweigh the samples results to mimic the voting. This practice is both controversial and necessary. Rarely disclosed Reweighing is considered a fraud by some and secrete sauce by others.

  3. Voter turn out: tough to predict who is going to show up at the voting booth. Young voters are notorious for this.

  4. Voters not willing to tell the pollsters the truth. Pollsters are generally not well likely. In races where some candidates are disparaged, voters may not be willing to say who they are voting for, or may intentionally mislead the pollsters.

  5. Biased pollsters: the pollsters may be subject to own biases and see the results not fairly, subconsciously or consciously.

  6. Echo chamber: some of the pollsters aren't that good to begin with. So they mimic the "herd" which forms a self fulfilling phenomenon.

  7. Polling business model; the pollsters have a challenging task at hand. They want to be right to be credible. Yet, they want to sell their results to their customers - mostly newspaper, television, ..., mostly left leaning media outlets. You will observe that polling usually favors liberal candidates and the gap narrows towards the voting date.


In sure you can come up with additional reasons as to why polling isn't reliable. That's why we haven't replaced actual voting with polling.

  • 1
    This explains some of the potential flaws in polling in general, but doesn't address the specific of the particular polling being asked. Despite these potential flaws, reputable election polling entities have done rather well in the past in terms of extrapolating poll data out into predictions.
    – user1530
    Commented Apr 21, 2017 at 17:22
  • Some of these seem to be written from the US perspective, in particular, most media outlets in the UK are right leaning
    – James K
    Commented Apr 27 at 10:23

The British Election Study commented on your first question.

In essence, pre-election polls got too large a proportion of responses from voters, hence over estimated the turnout, especially for young people who are more likely to answer surveys but less likely to turn out to vote than other groups.


The British election study had a survey of over 3000 adults which was pretty accurate as opposed to other polls, which suggests that the other polls were not nearly random enough to be accurate. So to make more accurate predictions for the 2017 election the people that they poll need to be selected more randomly.

  • But isn't there some inherent bias, for example people voting a certain way being more likely to refuse answering the poll ?
    – Bregalad
    Commented Apr 20, 2017 at 9:13
  • @Bregalad Are you alluding to the "Shy Tory/Shy UKIP/Shy *) factor? en.wikipedia.org/wiki/Shy_Tory_Factor It's worth noting that an enquiry by the British Polling Council found that this wasn't a factor in the 2015 General. Commented Apr 20, 2017 at 10:25
  • Well, in an hypotetical case were 50% of people will vote for A and 50% of people will vote for B, but 60% of people going to vote for A will refuse to answer a poll and only 40% of people voting B will refuse to answer. The poll will predict B will win over A because more people voting for B answered the poll, but the poll was biased in the 1st place, because the pool that answered the poll weren't representative of the pool of voters.
    – Bregalad
    Commented Apr 20, 2017 at 10:31
  • Interesting point, though I'm almost certain that polling groups will keeping asking the questions until they get responses from the number of people in each "sample group" would this not eliminate that bias? Commented Apr 20, 2017 at 10:51
  • @Bregalad - You are describing self-selection bias. Commented Apr 24, 2017 at 19:03

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