Banzhaf Power Index

The number of ways in which a group of $n$ with weights $\sum_{i=1}^n w_i=1$ can change a losing coalition (one with $\sum w_i<1/2$) to a winning one, or vice versa. It was proposed by the lawyer J. F. Banzhaf in 1965.

References

Paulos, J. A. A Mathematician Reads the Newspaper. New York: BasicBooks, pp. 9-10, 1995.