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Posted by on Oct 10, 2007

Michael Gallagher as mathematician

Searching for myself on Google, as I am prone to do, I came across the Wikipedia entry for another Michael Gallagher, the one at Trinity College, Dublin, who seems to be reasonably intelligent, thus debunking my world view of the Gallagher name. The entry is below, along with the equation above:

The Gallagher Index (or least squares index) is used to measure the disproportionality of an electoral outcome, that is the difference between the percentage of votes received and the percentage of seats a party gets in the resulting legislature.

This is especially useful for comparing proportionality across electoral systems. The index involves taking the square root of half the sum of the squares of the difference between percent of vote and percent of seats (as whole numbers) for each of the political parties.

The index weighs the deviations by their own value, creating a responsive index, ranging from 0 to 100. The lower the index value the lower the disproportionality and vice versa. Michael Gallagher, who created the index, included ‘other’ parties as a whole category, and Arend Lijphart modified it, excluding those parties.

Unlike the well-known LoosemoreHanby index, the Gallagher index is less sensitive to small discrepancies.
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