See the Banzhaf power index
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The Banzhaf power index, named after John Banzhaf (originally invented by Lionel Penrose in 1946 and sometimes called Penrose–Banzhaf index; also known as the Banzhaf–Coleman index after James Samuel Coleman), is a power index defined by the probability of changing an outcome of a vote where voting rights are not necessarily equally divided among the voters or shareholders.
To calculate the power of a voter using the Banzhaf index, list all the winning coalitions, then count the critical voters. A critical voter is a voter who, if he changed his vote from yes to no, would cause the measure to fail. A voter's power is measured as the fraction of all swing votes that he could cast.
An example is imagine a country with three states.
Two large states A and B with 49 votes each, and one small state C with only 2 votes.
Which states have the most power?
If A and B agree, then they will always get their way with 98 votes.
But if they disagree (49-49) suddenly the state with 2 votes, decides the issue.
Similarly if A and C agree, or B and C agree, then the other big state is powerless to change the outcome.
This actually makes the small state C, the most powerful.