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Mathematics LibreTexts

3.3: A Look at Power

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    Consider the voting system \([10: 11, 3, 2]\). Notice that in this system, player 1 can reach quota without the support of any other player. When this happens, we say that player 1 is a dictator.

    Dictator

    A player will be a dictator if their weight is equal to or greater than the quota. The dictator can also block any proposal from passing; the other players cannot reach quota without the dictator.

    In the voting system \([8: 6, 3, 2]\), no player is a dictator. However, in this system, the quota can only be reached if player 1 is in support of the proposal; player 2 and 3 cannot reach quota without player 1’s support. In this case, player 1 is said to have veto power. Notice that player 1 is not a dictator, since player 1 would still need player 2 or 3’s support to reach quota.

    Veto Power

    A player has veto power if their support is necessary for the quota to be reached. It is possible for more than one player to have veto power, or for no player to have veto power.

    With the system \([10: 7, 6, 2]\), player 3 is said to be a dummy, meaning they have no influence in the outcome. The only way the quota can be met is with the support of both players 1 and 2 (both of which would have veto power here); the vote of player 3 cannot affect the outcome.

    Dummy

    A player is a dummy if their vote is never essential for a group to reach quota.

    Example 2

    In the voting system \([16: 7, 6, 3, 3, 2]\), are any players dictators? Do any have veto power? Are any dummies?

    Solution

    No player can reach quota alone, so there are no dictators.

    Without player 1, the rest of the players’ weights add to 14, which doesn’t reach quota, so player 1 has veto power. Likewise, without player 2, the rest of the players’ weights add to 15, which doesn’t reach quota, so player 2 also has veto power.

    Since player 1 and 2 can reach quota with either player 3 or player 4’s support, neither player 3 or player 4 have veto power. However they cannot reach quota with player 5’s support alone, so player 5 has no influence on the outcome and is a dummy.

    Try it Now 2

    In the voting system \([q: 10, 5, 3]\), which players are dictators, have veto power, and are dummies if the quota is 10? 12? 16?

    Answer

    In the voting system \([q: 10, 5, 3]\), if the quota is 10, then player 1 is a dictator since they can reach quota without the support of the other players. This makes the other two players automatically dummies.

    If the quota is 12, then player 1 is necessary to reach quota, so has veto power. Since at this point either player 2 or player 3 would allow player 1 to reach quota, neither player is a dummy, so they are regular players (not dictators, no veto power, and not a dummy).

    If the quota is 16, then no two players alone can reach quota, so all three players have veto power.

    To better define power, we need to introduce the idea of a coalition. A coalition is a group of players voting the same way. In the example above, \(\left\{P_{1}, P_{2}, P_{4}\right\}\) would represent the coalition of players 1, 2 and 4. This coalition has a combined weight of \(7+6+3 = 16\), which meets quota, so this would be a winning coalition.

    A player is said to be critical in a coalition if them leaving the coalition would change it from a winning coalition to a losing coalition. In the coalition \(\left\{P_{1}, P_{2}, P_{4}\right\}\), every player is critical. In the coalition \(\left\{P_{3}, P_{4}, P_{5}\right\}\), no player is critical, since it wasn’t a winning coalition to begin with. In the coalition \(\left\{P_{1}, P_{2}, P_{3}, P_{4}, P_{5}\right\}\), only players 1 and 2 are critical; any other player could leave the coalition and it would still meet quota.

    Coalitions and Critical Players

    A coalition is any group of players voting the same way.

    A coalition is a winning coalition if the coalition has enough weight to meet quota.

    A player is critical in a coalition if them leaving the coalition would change it from a winning coalition to a losing coalition.

    Example 3

    In the Scottish Parliament in 2009 there were 5 political parties: 47 representatives for the Scottish National Party, 46 for the Labour Party, 17 for the Conservative Party, 16 for the Liberal Democrats, and 2 for the Scottish Green Party. Typically all representatives from a party vote as a block, so the parliament can be treated like the weighted voting system:

    \([65: 47, 46, 17, 16, 2]\)

    Solution

    Consider the coalition \(\left\{P_{1}, P_{3}, P_{4}\right\}\). No two players alone could meet the quota, so all three players are critical in this coalition.

    In the coalition \(\left\{P_{1}, P_{3}, P_{4}, P_{5}\right\}\), any player except \(P_1\) could leave the coalition and it would still meet quota, so only P1 is critical in this coalition.

    Notice that a player with veto power will be critical in every winning coalition, since removing their support would prevent a proposal from passing.

    Likewise, a dummy will never be critical, since their support will never change a losing coalition to a winning one.

    Dictators, Veto, and Dummies and Critical Players

    A player is a dictator if the single-player coalition containing them is a winning coalition.

    A player has veto power if they are critical in every winning coalition.

    A player is a dummy if they are not critical in any winning coalition.


    3.3: A Look at Power is shared under a CC BY-SA 3.0 license and was authored, remixed, and/or curated by David Lippman via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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