Shapley–Shubik power index

This article is about the index in simple voting games. For the broader cooperative-game solution concept, see Shapley value.

The Shapley–Shubik power index (SSPI) is a measure of a participant’s a priori voting power in a decision-making body. It was introduced by Lloyd S. Shapley and Martin Shubik in 1954 as an application of the Shapley value from cooperative game theory to simple yes–no voting games. The index assigns to each player the probability of being pivotal—that is, the first to turn a losing coalition into a winning one—over all possible orderings of voters.

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