Polling Medians and Accuracy

Past elections do not suggest the polling median is “highly likely to be accurate”.

As part of their amended article on BBC election night coverage and bias, Prof Norris and Prof Dunleavy maintain:

Any political scientist could have told the BBC that the median result here was highly likely to be accurate on national vote share.

This article examines that claim.

Predictive Power?

Numerous market research companies attempt to estimate the opinion of the British public, and ask people how they intend to vote in upcoming elections.

We can look at the performance of final polls from British Polling Council companies, in the last four General Elections:

“Highly likely” to be accurate?

The median is the middle value in a sorted list. The arithmetic mean sums up all the values and divides by how many values there are. It is notable that the mean and median of vote intention estimates are similar for the Conservatives, Labour and the Liberal Democrats.

Whilst the polls were broadly accurate in the 2019 General Election, this was not the case in either 2010, 2015 or 2017.

General Election 2010