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 in 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.
http://powerslave.val.utu.fi/ Power index site, with index calculators.