About Brecon and Consistency

Following an assertion on social media, I chose to look at two recent parliamentary by-elections for their consistency with national opinion polling.

The two latest by-elections in Peterborough and Brecon & Radnorshire both show consistency with timely opinion polls.

In Peterborough and Brecon

The Peterborough by-election was forced by the first successful recall of an MP under the Act of 2015. Fiona Onasanya was formerly a Labour MP — who had the whip removed prior to the successful recall — after being found guilty of perjury.

Ms Onasanya did not contest the seat, with Labour candidate Lisa Forbes becoming the new MP in the by-election on 6th June 2019.

We can compare Westminster opinion polls to Prof Hanretty’s constituency mapping of the 2019 European elections. Simplistically applying a uniform swing to the constituency projection essentially recovers the by-election result.

Polling data tables: YouGov 29th May, Opinium 30th May, Deltapoll 30th May, YouGov 6th June. YouGov included the Brexit Party and Greens in their primary prompt for the first time in their 5–6th June poll.

In the case of Brecon & Radnorshire, Chris Davies (then the Conservative MP) was recalled after making a false expenses claim. Mr Davies was then re-selected by the Conservatives to fight the by-election.

Additionally, neither Plaid Cymru nor the Greens put up candidates — directing their campaigners to support the Liberal Democrat candidate Jane…