Traditional measures of an election’s fairness have focused on party proportionality: does the proportion of votes party X earns match the proportion of seats party X is awarded by the electoral system? This is measured by the Gallagher Index, the Loosemore-Hanby Index, and others.
This paper proposes a complementary voter-centric measure, the Legislative Power Disparity Index, which measures the fairness of an election as a whole from the voter’s perspective. It is built on a Legislative Power Score (LPScore), which is assigned to each voter in the election. In a perfect world each voter would have an LPScore of 1 and the election as a whole would have an Legislative Power Disparity Index (LPDI) of 0.
Of course, we do not live in a perfect world. We describe the application of Legislative Power to what we believe would be typical of real-world Canadian elections. Unfortunately, our current Single Member Plurality (sometimes called First-Past-The-Post) does not fare well. When other common electoral systems such as List Proportional Representation, Single Transferable Vote, and Mixed Member Proportional are considered, the Legislative Power Disparity Index shows significant differences between them that are not revealed by the traditional party proportionality measures.