Wrong again: Opinion polls had suggested that the "Remain" side would win. DW spoke with John Curtice, a British professor of politics who specializes in opinion polls, about what might have happened.
DW: The outcome of the Brexit vote came as a surprise to many after opinion polls had seemed confident that the "Remain" side would win. Following the Conservatives' surprise general election win last May, this is the second time in a relatively short period that pollsters in Britain got it wrong. Are such surveys still reliable?
Professor John Curtice: You're somewhat exaggerating the error in the polls: Two of the final polls that were published did actually have "Leave" ahead, at 51-49. On average, the polls had the "Remain" side ahead, but, in truth, pretty narrowly. I think there are some questions there for the pollsters to answer. In particular they may wish to review some of the decisions made in the last couple of weeks in the campaign about changing the way they were waiting and sourcing the polls, all of which tended to push the "Remain" vote up. What's also true is that many people misread the polls; there was a tendency to look at the polls that were consistent with what they hoped would happen and to ignore those that weren't.
Were you surprised at the outcome?
Personally, no. For much of the campaign, I thought the balance was in favor of "Remain," but it was only a question of "Remain" might win on a balance of probabilities: It was never ever a certainty. I certainly became less optimistic for the "Remain" side during the last couple of weeks because it was quite clear that it was the "Leave" side that won the campaign. ... Maybe the "Remain" side were very, very slightly favored, but I think the point was that it was close to a 50-50 shot. So, surprised, no.
Do you think the murder of Member of Parliament Jo Cox influenced pollsters who saw "Remain" ahead?
To be honest, we'll never know what the implications of the tragic murder of Jo Cox were. All one can say is the polls that were conducted after her murder were better for "Remain" than those that were conducted before, although there were some signs of it happening anyway, so that can not necessarily be ascribed to the murder. Pollsters by that stage were engaged in a serious effort to try to gauge public opinion as accurately as possible.
Why and to whom do opinion polls matter so much?
They matter to people on the financial markets. As we've seen today, the financial markets seemed to have decided to follow the polls that they hoped were right - and as a result that "Remain" was going to win - and then have found themselves with a very different scenario.
In the absence of opinion polls, what you get are unsubstantiated assertions by the protagonists in the campaign as to what the public think. And, although opinion polls may not be entirely accurate, the errors we're talking about here are relatively small ones. It's still better to have that information critically evaluated than no information at all - and therefore a free run for every politician to make claims that nobody can check.
John Curtice teaches politics at the University of Strathclyde in Glasgow. He has written several books, including "Playing with Political Fire: Cameron, the Conservatives and the EU Referendum" and has won the International Journal of Market Research Collaborative Research Award.