The 2015 election has been described as both one of the most disproportional elections ever and one of the least. Alan Renwick discusses the notion of electoral disproportionality and weighs up the relative merits of the different indexes that have been developed to measure it.
In a report published earlier this month, the Electoral Reform Society declares the 2015 general election ‘the most disproportional election to date in the UK’. The ERS’s website cranks up the rhetoric further: ‘It’s official: this election was the most disproportionate in UK history.’ In marked contrast, my own first cut at analysing the election results, published a few weeks earlier, said that this was not the UK’s most disproportional election: that, indeed, it was the least disproportional since 1992.
So what is going on here? Which of us is right?
The simple answer to the first of those questions is that we have used different methods to measure proportionality. The simple answer to the second question is that there is no simple answer: it very much depends on what you mean by disproportionality.