NEWS31 March 2016

Unrepresentative samples led to GE polling miss, inquiry concludes

News Public Sector UK

UK — The report of the inquiry into the 2015 general election opinion polls has highlighted unrepresentative samples as the primary cause of the failure to predict the Conservative victory. 

Polling crop

The inquiry, jointly sponsored by the British Polling Council (BPC) and the Market Research Society (MRS), was chaired by Professor Patrick Sturgis, director of the National Centre for Research Methods.

The resulting report, released today, concludes that "the methods the pollsters used to collect samples of voters systematically over-represented Labour supporters and under-represented Conservative supporters". It adds that the statistical adjustment procedures applied to the raw data "did not mitigate this basic problem to any notable degree", and that, since the inquiry replicated all published estimates for the final polls using raw micro-data, the use of flawed analysis or inaccurate weighting targets could be discounted.

Procedures used to handle postal voters, overseas voters and un-registered voters were also found to have made no detectable contribution to the polling errors. 

The report makes 12 recommendations on the basis of its findings, including that pollsters should include questions during the short campaign to determine whether respondents have already voted by post; review existing methods for determining turnout probabilities; review current allocation methods for respondents who say they don't know, or won't disclose, who they intend to vote for; and take measures to obtain more representative samples within the weighting cells they employ.

It also recommended that the Economic and Social Research Council should fund a pre- and post-election random probability survey as part of the British Election Study in the 2020 election campaign.  

“We would like to offer our thanks to Prof. Patrick Sturgis and his panel for undertaking such a robust investigation, and we welcome their final report, published today," said Jane Frost, chief executive of MRS. "We will do everything we can to support our members in implementing the panel’s recommendations.”

The full report can be accessed here