Friday 8 May 2015

[4] FPTP bias - who's gained and lost the most in GE2015 due to FPTP?

The First-Past-The-Post method (as currently used e.g. in the UK parliamentary elections) has been heavily criticised (a booka very accessible video from CGP Grey), and understandably so, as it creates various problems with tactical voting, and the views of people not being accurately represented. There is a society which campaigns for electoral reform in the UK.

But let's look at the data - which party has gained / lost the most due to FPTP vs. proportional representation in the 2015 General Elections in the UK? What follows is my quick-and-dirty analysis.

[download the data and code]

The method I used is simple - I first calculated the proportion of seats each party gained in the last election. Then I subtracted from it the party's share of votes. To make the data comparable across parties, I divided that number by the share of votes.

UKIP is the biggest loser here, getting just one Commons seat with a 12.7% vote share (~98.8% loss). Democratic Unionist Party (NI) gained more than twice as many seats as its vote share would suggest (~105.1% gain).

There is no obvious relation between the vote share (or seats share) and how much the party gains or loses, so no obvious advantage to small / large parties. However, small regional parties (vide: SNP victory this election) seem to benefit the most.

Still, the number of votes cast seems to be a good predictor of the number of seats.
Unless you are UKIP. We don't like UKIP.

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