Evolving Shares

How did vote shares changes evolve throughout the UK election night?

As part of the BBC’s coverage, the broadcaster shows the actual vote share changes from constituencies which had declared their results.

The House of Commons Library has collated the BBC declaration times, so we can replicate the election night calculations.

The moving line

One response to a previous article was that vote share changes shown at around 2am on the BBC coverage were ‘understating’ Labour support.

This is a misunderstanding.

The Labour changes at this time are not ‘understating’ their support, nor is the Brexit Party being ‘overstated’. Of the constituencies that had declared on 12–13th December: the actual vote shares for each party were aggregated from the 2017 General Election, and then compared to the declared results.

In other words, the BBC graphic is showing the actual vote share change for all declared constituencies.

Declaration times are not uniform. For example, the first six constituencies to declare were all in North East England (Newcastle Upon Tyne Central, Houghton and Sunderland South, Blyth Valley, Newcastle Upon Tyne East, Sunderland Central, and Middlesborough).

Changes in support are not uniform across the country. (Image: House of Commons Library)

Labour’s aggregate net change in vote share in those six constituencies was -13 points. This is compared to the party having a net loss of eight points across the whole of Great Britain.

As constituencies declared, the calculation changes:

The step-changes are more dramatic earlier in the night, as fewer constituencies have declared. (Image: ggplot2)

Using the gganimate package in R, we can create an animated graph showing that progression:

Nine constituencies in Great Britain declared after 6am on 13th December. (Image: gganimate)

The House of Commons Library has collated the BBC declaration times and results for the 2017 and 2019 General Elections. The R code and Excel files may be found on GitHub, and there is a R Markdown web page.

This blog looks at the use of statistics in Britain and beyond. It is written by RSS Statistical Ambassador and Chartered Statistician @anthonybmasters.

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