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Version 1 (Panagiotis Louridas, 05/07/2010 03:30 pm)

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h1. Calculation of Voting Power
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This is a project hosting software for calculating the voting power of voters with weighted votes in an assembly.
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The software is open source and released under the terms of the "BSD license":http://www.opensource.org/licenses/bsd-license.php.
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h1. Overview
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A common error is the assumption that, in weighted voting, the power of a voter is their voting weight. In reality, although the weight of a voter is important, we must take into account the weights of the other voters, as well as the decision making rule, in order to arrive at a conclusion about the actual voting power each participant in the vote has.
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The term _voting power_ refers to an index that captures the power of a voter to influence the outcome of a voting process. There have been several definitions of voting power in the literature, although they remained outside the discussions of mainstream politics until late in the 20th century; what's more, definitions of voting power were proposed, forgotten, and reinvented during the years. Perhaps the most intuitive definition is the one given by "Lionel Sharples Penrose":http://en.wikipedia.org/wiki/Lionel_Penrose in 1946; according to this definition, the voting power of a voter is the probability that they can swing the vote. This is known as the Penrose index or the "Penrose Banzhaf index":http://en.wikipedia.org/wiki/Penrose%E2%80%93Banzhaf_index, after "John Francis Banzhaf III":http://en.wikipedia.org/wiki/John_F._Banzhaf_III who reinvented the index in 1965.
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The index is as follows:
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* Suppose we have _n_ voters, each with a specified weight.
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* The voters are free to vote as they please.
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* All votes are _yes_ or _no_.
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* There is a rule that decides when a vote passes; it may be 50% of the weights of the proponents (simple majority), or some other figure; in addition, it may require that a number of voters also support (a given quorum).
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* A voter is critical in a vote if without them the vote does not pass, but with them it passes. That is, the vote must be a swing vote, decided by the voter.
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* Count the number of critical votes for a voter.
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* Take all possible partitions of voters in yes and no camps.
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* Divide the number of critical votes for a voter by the number of all possible voter partitions.
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* The result is the probability that the voter can swing a vote.
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The above definition does not lead to a practical procedure for non-toy problems, as suffers from combinatorial explosion regarding the enumeration of swings and the partitions; hence, one has to find an alternative method to calculate it for many real-world assemblies.
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h1. The Software
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The software consists of a single Perl script that calculates the Penrose index