The Inquiry established to investigate inaccuracies in the opinion polls during the 2015 General Election campaign will present its preliminary findings in detail today at the Royal Statistical Society in London.
Chaired by Patrick Sturgis, Southampton Professor of Research Methodology, the independent inquiry was set up last May by the British Polling Council (BPC) and the Market Research Society (MRS). The Inquiry panel, which includes Will Jennings, Southampton Professor of Political Science and Public Policy, was tasked with establishing the extent of inaccuracies in the polls, the reasons for inaccuracies, and whether the polls were adequately communicated to the general public.
The 2015 pre-election polls significantly under-estimated the size of the Conservative lead over Labour, with all the final polls suggesting a statistical ‘dead heat’. Following in-depth investigations, the Inquiry panel has concluded that the primary cause of the failure of the polls was unrepresentativeness in the composition of the poll samples. The methods of sample recruitment used by the polling organisations resulted in systematic over-representation of Labour voters and under-representation of Conservative voters. Statistical adjustment procedures applied by polling organisations were not effective in mitigating these errors.
The Inquiry has also reached this conclusion by determining that other potential causes were likely to have made, at most, a modest contribution to the total error. These factors include voter turnout misreporting; question wording and ordering; treatment of overseas voters; treatment of postal voters; and treatment of un-registered voters.
The Inquiry will make a number of recommendations to the BPC and MRS when it publishes its report in March.
Professor Sturgis has also appeared in the first of a three-part BBC Radio 4 series examining the role of opinion polling in British politics entitled ‘Can We Trust the Opinion Polls?’ which looks at the track record of polls in previous elections. The first episode, available via BBC iplayer radio, was broadcast on Sunday 17 January.