Gallagher Disproportionality Index

The Gallagher disproportionality index, also called the least squares index (LSq), is the standard summary measure of how faithfully an electoral system translates votes into seats. Introduced by Michael Gallagher in 1991, it takes the difference between each party's vote share and its seat share, squares those differences, sums and halves them, and takes the square root. Because deviations are squared before aggregation, the index gives disproportionate weight to a few large discrepancies rather than many small ones, capturing the intuition that one badly over- or under-represented party distorts the result more than scattered rounding errors. It has become the most widely reported single-number diagnostic of electoral-system performance in comparative political economy.