[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 book, a 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.