What Is the Gallagher Index for a BMP Parliament?

There has been a lot of talk about the Gallagher Index in the context of electoral reform. The Gallagher Index is a measure of the relative proportionality of different electoral systems. It compares how well the share of the vote is translated into share of seats and ranges from zero (for perfect proportionality) upwards.

The current Parliament, elected in 2015 by First-Past-The-Post, has a Gallagher Index of 12.0. By contrast, a Bicameral Mixed-member Proportional Parliament would have had a Gallagher Index of 4.1. The Special Committee on Electoral Reform has recommended that our electoral system should have a Gallagher Index of 5 or less.

The chart below shows how the Gallagher Index would have varied over the last seven elections for BMP compared to FPTP. BMP yields a much lower Gallagher Index, which has been below the threshold recommended by the committee for the last five elections.