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MAT 105 Spring 2008

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In many voting systems, the voters are not treated equally Juries: If one voter votes “not guilty,” then the result is “not guilty” Stockholders: If you have more shares of stock, then your vote is weighted more heavily US Electoral College: Larger states get more votes European Union: Larger member countries get more votes

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Each participant has a specified number of votes, called his or her weight For simplicity, we will assume all elections are “yes” or “no” There is a criterion for determining whether “yes” or “no” wins: the quota

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To pass a bill in the House of Representatives, you just need a majority of the votes to be “yes” There are 435 total votes, so the quota is 218 To amend the US Constitution, 3/4 of the states must ratify the amendment There are 50 states, so the quota is 38

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In US Presidential elections, plurality elections are held in each state The candidate winning each state sends electors to vote for him or her in the Electoral College The number of electors per state depends on the size of the state: bigger states have more electors

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When we talk about weighted voting systems, we use a compact notation to list the quota together with the various weights [q; a, b, c, …] q is the quota a, b, c, etc. are the weights So the electoral college system would be written [270; 55, 34, 31, 27, 21, 21, etc.]

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As we said before, to simplify things, our elections will always be deciding “yes” versus “no” This isn’t so unreasonable, since the most common place to find weighted voting systems is in legislatures or other government bodies We say that the voters cast their votes “in favor” or “against” a motion by voting yes or no

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A dictator is a voter with all the power: a motion will pass only if the dictator votes in favor, and it doesn’t matter how the other participants vote [51; 60, 40] In this system, the weight-60 voter is a dictator [20; 15, 10, 5] In this system, none of the voters is a dictator

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A dummy voter is a voter whose vote does not matter. When voters form a coalition to vote in favor or against a motion, the dummy voter can be removed from the coalition without changing the result [51; 26, 26, 26, 22] The voter with weight 22 is not needed to win when two others combine to support a motion, and she does not have enough weight to pass a motion with only one other

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A voter whose vote is necessary to pass any motion is said to have veto power [21; 20, 15, 5] The voter with weight 20 has veto power: if that voter votes “no,” then the motion cannot pass Jury: [12; 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1] Since all 12 voters must vote in favor to pass the motion, each voter has veto power

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[8; 5, 3, 1] None of the voters is a dictator The weight-1 voter is a dummy voter The weight-5 and weight-3 voters have veto power [9; 5, 3, 1] Motions can only pass unanimously: none of the voters is a dictator, there are no dummies, and all of the voters have veto power

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[6; 5, 3, 1] None of the voters is a dictator None of the voters is a dummy The weight-5 voter has veto power [51; 49, 48, 3] None of the voters is a dictator None of the voters is a dummy None of the voters has veto power

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Consider the system [51; 49, 48, 3] Even though the third participant only has a weight of 3, it has the same “power” as the other two Any two of the three participants can combine to pass a motion, and none of the three can pass a motion alone

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In the system [51; 26, 26, 26, 22], the fourth participant has almost as much weight as the other three, but is a dummy voter Dummies have no power to influence elections one way or the other

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We have seen that the power a voter wields is not necessarily directly related to the weight of the voter We will want to measure the power of each voter, keeping in mind that: dictators have all the power; when one voter is a dictator, all other voters are dummies dummies have zero power

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A voter is a dictator if it can pass a motion by itself; if a voter has at least as many points as the quota, it is a dictator A voter has veto power if it can defeat a motion by itself; if a voter has at least as many points as the total number of votes minus the quota, plus 1, then it has veto power A voter is a dummy if it can never be an essential member of a winning coalition; this is harder to detect, and we’ll be looking into this more soon

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