手法を比較

	Coalition Formation Analysis	Voting Power Index Analysis
分野	Political Economy	Political Science
系統	MCDM	MCDM
提唱年	1962	1954
提唱者	William Riker; Michael Laver & Norman Schofield	Lloyd Shapley & Martin Shubik; John F. Banzhaf III
種類	Formal theory of coalition selection	Cooperative game-theoretic measure of a priori voting power
原典	Riker, W. H. (1962). The Theory of Political Coalitions. Yale University Press. ISBN: 9780300001754	Shapley, L. S., & Shubik, M. (1954). A Method for Evaluating the Distribution of Power in a Committee System. American Political Science Review, 48(3), 787-792.
別名	Minimal Winning Coalition Theory, Riker Size Principle, Coalition Theory, Government Coalition Analysis	Voting Power Index, Shapley-Shubik Index, Banzhaf Power Index, A Priori Voting Power Analysis
関連	4	4
概要	Coalition formation analysis is the formal study of which subset of parties will combine to form a governing or decision-making coalition when no single party commands a majority. William Riker's 1962 The Theory of Political Coalitions supplied the foundational logic: under pure office-seeking, rational politicians form minimal winning coalitions and, by the size principle, the smallest winning coalition possible, so that the spoils of office are divided among as few partners as necessary. Michael Laver and Norman Schofield's 1990 Multiparty Government enriched this with policy-seeking motives, showing that coalitions also tend to be ideologically connected. The framework predicts coalition membership from seat shares and party positions.	Voting power index analysis measures the a priori capacity of each member of a weighted voting body to influence collective decisions, defined as the probability that the member is pivotal — that their vote turns a losing coalition into a winning one. The two canonical indices are the Shapley-Shubik index, introduced by Lloyd Shapley and Martin Shubik in 1954 as a specialization of the Shapley value to simple voting games, and the Banzhaf index, formalized by John Banzhaf in 1965. Both reveal that a player's share of power generally differs sharply from its share of votes.