Sure.
In the graphs we looked at earlier, there's always a gap between the percentage of votes and the percentage of MPs. The Gallagher index takes all of those gaps and it expresses them as a single number. Rather than saying, well, the Liberals are overrepresented by 15% and the Conservatives are under-represented by 7% or whatever, it takes those numbers and puts them into a single number. We can talk about how it does that if you want, but the best intent....
One of the downfalls of the Gallagher index is that overrepresentation, say, in Alberta, can be offset by overrepresentation in, say, the Maritimes. That does not show up in the classic Gallagher index, so I adapted it to what I call the composite Gallagher index. I calculate the Gallagher index for each region of the country and then average them together, so that disproportionalities in the Maritimes and disproportionalities in Alberta both contribute to an overall picture of how proportional the system is.