Seatvote

Seatvote is a term used in political science and electoral analysis to describe the relationship between the share of votes that political parties receive and the share of seats they obtain in a legislative body. The concept is closely associated with the seat-vote curve, a functional mapping from vote shares to seat shares that varies across electoral systems and districting arrangements. In practice, seatvote analysis combines historical election results, district maps, and vote distributions to estimate how votes translate into seats for each party and to measure disproportionality.

Analysts fit models that may assume uniform or heterogeneous swings across districts, and they may use different seat allocation rules (such as first-past-the-post, proportional representation with remaining-seat methods, or mixed systems) to predict seat outcomes under alternative vote patterns. Outputs typically include predicted seat shares for given vote shares, measures of disproportionality, and counterfactual scenarios under redistricting or electoral reform.

Seatvote is used in comparative politics, electoral reform debates, and forecasting to assess how changes in district boundaries, thresholds, or voting rules would affect representation. It can inform assessments of proportionality, responsiveness, and party-system dynamics, and it is often accompanied by visualization of the seat-vote curve and related indices (for example, the Gallagher index).

Limitations include sensitivity to model assumptions, data quality, and the presence of coalitions or strategic behavior that violate the model's assumptions. Seatvote remains a descriptive tool rather than a deterministic forecast, providing a framework to compare different electoral designs and to understand the mechanics of translation from votes to seats.