power index

Any attempt to measure the power of a voting bloc in terms of the likelihood that it will be the swing voter, able to decide whether a proposition wins or loses. The first formal power index was proposed by Lionel Penrose 1946 (although the idea was foreshadowed by the anti- Federalist Luther Martin in 1787). The best-known index is the Shapley-Shubik index. Unfortunately, different indices have different values in the same situation. Some critics deny that they have any meaning at all; supporters of the concept have been trying to produce a more general index, but none has caught on.