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Latest research reveals opinion poll errors are not increasing

Published: 16 March 2018
Big Ben at the Houses of Parliament
Study reveals no evidence to suggest that polling errors have increased over time

A new study led by the University of Southampton has revealed that claims of a crisis in the accuracy of election polling are false and that there is no evidence to suggest that poll errors have increased over time.

This latest research, led by Professor Will Jennings at the University of Southampton and Professor Christopher Wlezien from the University of Texas at Austin, and published in the journal Nature Human Behaviour, focussed on more than 30,000 national polls from 351 general elections in 45 countries over the period between 1942 and 2017.

Pre-election polls have sought to measure the general public’s preferences for candidates or parties for almost three quarters of a century. While the wording of survey questions can differ across countries, these polls are used to forecast election results.

The paper, titled ‘Election polling errors across time and space’, confirmed that the average absolute error of pre-election polls has fluctuated over the years but has not increased. When polling was first introduced during the 1940s and 1950s, the mean error was 2.1 per cent. This average was also consistent during the 1960s and 1970s and has been two per cent since 2000.

Moreover, in countries where there is regular pre-election polling over a period of almost forty years, the evidence is that polls have become more accurate, not less.

Professor Will Jennings, from the University of Southampton, said: “It is understandable that people tend to focus on high profile polling misses, and shock election results, but the evidence dispels the claim that polling is any more inaccurate that at any point in history. 

“Of course polling is a tricky business and has to adapt to changes in politics and wider society, and will inevitably go wrong from time to time. But it is important not to rush to judgment and generalise from individual cases. 

“Our study shows the importance of testing polling accuracy over the long-term and in cross-national perspective.”

The report shows that there is no significant trend of increasing polling inaccuracy in recent times. This implies that declining response rates and the growing variation in survey mode, sampling, and weighting protocols together have had little effect on the performance of pre-election polls, at least when taken together. 

The research found that while all polls contain error, it tends to be greatest for parties or candidates receiving a larger share of the vote, as was the case in the 2015 and 2017 UK general elections and the 2016 US presidential election.

Characteristics of political systems also appear to influence the accuracy of polling. Errors tend to be lower in proportional representation systems as well as presidential elections, particularly in the USA and France.


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