UK: Narrowing polls has increased the probability of alternative scenarios – Deutsche Bank

UK opinion polls have narrowed significantly as the election approaches, from highs of a 20 point Conservative lead over Labour to current poll average of 7pp at the time of writing and while current polling still remains consistent with the Conservatives increasing their current slim majority, the tightening of polls and their wide range has increased the likelihood of alternative election outcomes explains the research team at Deutsche Bank.

Key Quotes

“Following the sharp narrowing of the Conservative’s polling lead in recent weeks, the Conservatives’ lead over Labour is now down to 7 points. Comparing this to the historic margins of error seen over previous elections, current polling remains just in excess of the largest polling miss observed since 1992 – a 6.3pp overestimate of the Labour vote share in 2001 – and well in excess of the average polling error over these votes. At the same time, in elections since 1992 the pollsters have consistently underestimated the Conservative’s vote share, by 2.1pp on average.”

“Given this clear trend for polls to underestimate the Conservatives for various reasons, including issues of poor sampling together with the “shy Tory phenomenon” – pollsters attempt to correct their survey results to account for this dynamic. The risk therefore is that the polls could be over-correcting and therefore giving the Tories an advantage in the polls.”

“The raw voting intention data behind the most recent polls from each of the main pollsters is relatively similar, with a limited spread across companies. There is, however, a much larger dispersion of results when looking at the final headline poll lead that is published. Indeed, from an average Conservative lead of 1.3% based on the raw data, the lead increases to 6.3% after the polling agencies’ re-weighting, with a much wider spread.”

“In other words, the wide range in the headline Conservative lead across polls is a result of the statistical re-weighting employed by the polling agencies in an attempt to correct for previous Conservative polling misses rather than a result of variation in the raw survey data.”

“With the Tory poll lead ranging from +1 to +12 across different polls, the key question for election night will re-weighting will be most correct. Those companies reweighting the Conservative vote share the most in fact tended to better predict the 2015 final result.”

“Thus, while there is significant re-weighting of polls in favour of the Conservatives, this should not in itself imply the Conservative lead is being over-stated. Indeed, in 2015 the pollsters currently re-weighting the most – ComRes, IpsosMori and ICM – tended to get closest to the final result.”