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How accurate is election polling anyway?

With the British Polling Council dissection of what went wrong in the 2015 election imminent, I’ve taken a look at an even more important underlying question: how accurate is election polling anyway? As the charts show, it’s not great…

How good are the polls anyway LAB CON-01

I’ve been working on this for a week or so now (my laziness means that some people have gotten there ahead of me). I’m keen to try and make this as fair as possible, so I asked a pollster what they considered the margin of error to be on their headline figures. The results staggered me – they said +/-3%, a margin so high it covers the whole distance between the Conservative and Labour vote shares at the 2015 election. It’s fair to say I’m not willing to be THAT fair. As an additional piece of housekeeping, it’s worth noting that poll results are generally rounded to the nearest whole number, whilst election results are rounded to one decimal place, allowing a potential 0.5% margin of error per party, which I’ve displayed on the three charts.

How good are the polls anyway LAB CON LIB-01

The question is though, if the polls haven’t been great, why is it only now that people are getting worked up about it? The answer is ultimately quite simple – polling companies have been getting off easy because people haven’t been looking to the polls to be completely accurate, but simply to inform people which party is going to win. That’s why the frankly not amazing accuracy of, say the 1997 election, has been overlooked. And things are only going to get worse for pollsters…

How good are the polls anyway LAB CON LIB OTHERS-01

Why? Because of the rise of smaller parties. One of the major problems with British opinion polling is that because of the first-past-the-post system, vote share can have very little relation to the number of seats, and the disruption caused by minor parties will exacerbate this.



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