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7.2: Weighted Voting

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    Voting Power

    There are some types of elections where the voters do not all have the same amount of power. This happens often in the business world where the power that a voter possesses may be based on how many shares of stock he/she owns. In this situation, one voter may control the equivalent of 100 votes where other voters only control 15 or 10 or fewer votes. Therefore, the amount of power that each voter possesses is different. Another example is in how the President of the United States is elected. When a person goes to the polls and casts a vote for President, he or she is actually electing who will go to the Electoral College and represent that state by casting the actual vote for President. Each state has a certain number of Electoral College votes, which is determined by the number of Senators and number of Representatives in Congress. Some states have more Electoral College votes than others, so some states have more power than others. How do we determine the power that each state possesses?

    To figure out power, we need to first define some concepts of a weighted voting system. The individuals or entities that vote are called players. The notation for the players is \(P_{1}, P_{2}, P_{3}, \dots, P_{N}\), where \(N\) is the number of players. Each player controls a certain number of votes, which are called the weight of that player. The notation for the weights is \(w_{1}, w_{2}, w_{3}, \dots, w_{N}\), where \(w_1\) is the weight of \(P_1\), \(w_2\) is the weight of \(P_2\), etc. In order for a motion to pass, it must have a minimum number of votes. This minimum is known as the quota. The notation for quota is \(q\). The quota must be over half the total weights and cannot be more than total weight. In other words:

    \[\frac{w_{1}+w_{2}+w_{3}+\cdots w_{N}}{2}<q \leq w_{1}+w_{2}+w_{3}+\cdots+w_{N} \nonumber \]

    The way to denote a weighted voting system is \(\left[q: w_{1}, w_{2}, w_{3}, \dots, w_{N}\right]\).

    Example \(\PageIndex{1}\): Weighted Voting System

    A company has 5 shareholders. Ms. Lee has 30% ownership, Ms. Miller has 25%, Mr. Matic has 22% ownership, Ms. Pierce has 14%, and Mr. Hamilton has 9%. There is a motion to decide where best to invest their savings. The company’s by-laws define the quota as 58%. What does this voting system look like?


    Treating the percentages of ownership as the votes, the system looks like: \([58: 30,25,22,14,9]\)

    Example \(\PageIndex{2}\): Valid Weighted Voting System

    Which of the following are valid weighted voting systems?

    1. \([8: 5,4,4,3,2]\)

    The quota is 8 in this example. The total weight is ZULxcUFFGElQVARvTj6zv65_cGh4geck9SC8zuneAsmmDy90ZKDVmu6VE9Ni8O1mTpHJQlHzBivxBUZqJOH2iDDO5BnXW606NcoUVVc2mk_qA7pw6TRQ3qCrQkXXtvBQPKrx3Eg. Half of 18 is 9, so the quota must be LYyZpiQlXZF9akovSQV-wdyVQAZ-rxqWZbss8gsKprumQc7cw-crrT1e6Dw6evqytTabnaTyFt0WVSjjGWibeLqHoU41E9GQ6qVp0PdSlR2sRqt-O64O2VhUqBoCMn_kEdVa_V8. Since the quota is 8, and 8 is not more than 9, this system is not valid.

    1. \([16: 6,5,3,1]\)

    The quota is 16 in this example. The total weight is FE3w0-N3wvfu01WwjBXTswCEgxTJX-0SVrboH_KPVtx-9JIQtHJ9ruakzscSNgHSpfxLu_5mdMM_fDR2uosJXK4Ds1-OxvelxFIkLIXdf4UQUVcad97sB-pHL71QnEnkJQyxiGQ. Half of 15 is 7.5, so the quota must be 96-cYgIq678D5M_IyEqn3kYrNNpRIZ8RIg68aW0XOaiAUlrX2y_qfwbZixTQqZVRyikDyhnB1Vt8SvebTrq89yjykKAzYKldspnfXMvwLRLclGX6aA67S507xxQsBynqrL_Rey4. Since the quota is 16, and 16 is more than 15, this system is not valid.

    1. \([9: 5,4,4,3,1]\)

    The quota is 9 in this example. The total weight is dDII_C7s4WbkWXNOdU9DJNg4yJixkaMMGvYaXIja-aYY9eNnTCY-UUf6tjsFKikCxyCf-Zrnxg2k2PlPKWf5XbRClHemvDasZczCW4SQ-CI8mYH5P31bowhuI_TtPtDlxsb-iDU. Half of 17 is 8.5, so the quota must be sF6HYwmigultGxfMMYKcoQUYs8hY_GJIlEx7Wqd2da7levLN_4Zen4oDV0ZRNr0MhCyfAlVhCPz4mOTo9LXg1mj6XQCn8bijP07mv01bCHChGSRFmS_sqUDSOc21ug1d3CD9wVU. Since the quota is 9, and 9 is more than 8.5 and less than 17, this system is valid.

    1. \([16: 5,4,3,3,1]\)

    The quota is 16 in this example. The total weight is m-cHchvebHWXE_3AytqcUBJI9f2xB-MMSpQfhUY7rR5itmWSC35dWYF2MEbGkdZmvc37swICacHX2jdanhP6yeUSlVxAyQ1BCriV5yWKxEwZpkEMNiprx0h6Kx8yZlCDCXk3j-0. Half of 16 is 8, so the quota must be R0rE9ikMFYz2il6GYsNfI7P8RVTfHPfUWJisUKJzIKOTPW1AOnufLs0b60HFb3db__2kbauVK-wyaHxxmUdQ_Np8O542x9tnkRn618JhBX64tzk9mYNXM5QUNc8X0StqcSipfOM. Since the quota is 16, and 16 is equal to the maximum of the possible values of the quota, this system is valid. In this system, all of the players must vote in favor of a motion in order for the motion to pass.

    1. \([9: 10,3,2]\)

    The quota is 9 in this example. The total weight is vC0rkL4kOljMULysejlDSA2-hgToz8yMwRg6RfrBs5iMpDUrO-o7DvnrEvGdFGeYibrzWbgRe4WEr4x5zczN_eYHRmOmfm1gC8iHvNSuU3f-wum-nTF64J1pYo-kGHrfEObudvc. Half of 15 is 7.5, so the quota must be JXe-lrw3vZ4zb8OIpdnQ2UdpouEyMfU0bCIFsBpr3HCjkCJpy1ae2w-EKftDrrTJMbSloXKmwcEzFfm-hCFhzXhjVumqOlh78oajhzr-vVW7IozzHaVOgMWMvA1sssIS6gug9dY. Since the quota is 9, and 9 is between 7.5 and 15, this system is valid.

    1. \([8: 5,4,2]\)

    The quota is 8 in this example. The total weight is ifuwIb5rcfME2DhoHhOBZ1y0hvm6jkO8TKWo3dqguggeQazl8LtrSDMVadtBVcLwZx_82eIx3MmdsXMEOBnditHsKSAOcGdpDXPLojSHqKJYguqZ_MEngauLhsT60K_hAiGQd3Q. Half of 11 is 5.5, so the quota must be 4QxNQo_TQXb_ldXk-fShsRFsAQydG4rptsQsQctv2lMaUzRTJ7GoWJhHvFCFO5QFs3HlkTZLvT4xoiI2h9QF5e2xPx7aT8J_5SIpKg-0yYOdjIjo0AAyoIJLFj2fQx81WIM_xnA. Since the quota is 8, and 8 is between 5.5 and 11, the system is valid.

    In Example \(\PageIndex{2}\), some of the weighted voting systems are valid systems. Let’s examine these for some concepts. In the systemwHkD3NOU-Ij5vHZEvTGFSAeevUccnLEUMclvu1O1GpekOHmAONsYbePg0t-mLGaMiynd-0IprlTH6JwQ2QlY3WC_8Qy-eAgQTqYdAhKQjJSgDweT96yJ6HZ0Emqg6stF1rJ0uls, player one has a weight of 10. Since the quota is nine, this player can pass any motion it wants to. So, player one holds all the power. A player with all the power that can pass any motion alone is called a dictator. In the system 1b3fUe-wY6lZafgToRP-2x81_bkBfdm5kPiFFxdkfCK1bKFXIEQap9QlnU4_L8c0tUUY6wvtiMrJd_F1E28JGcxSqOqk3XPvh7Q4nXKJ7FB3XQYyYd7nvy4Zt6TFJLp0ftOKioY, every player has the same amount of power since all players are needed to pass a motion. That also means that any player can stop a motion from passing. A player that can stop a motion from passing is said to have veto power. In the system 4Wo4-o9-LmvWHiUru1IyjX9uD1ia_pNJh4V3Xu9A6XYq7tZxelNn44Voz9x20l9qr5RRUCkOEBGF1DqhJ5wzkvX7P9ws7Ydf8wRADKOjRNo2M1R-Cd2V2pBwsG6ehZOffr66Dak, player three has a weight of two. Players one and two can join together and pass any motion without player three, and player three doesn’t have enough weight to join with either player one or player two to pass a motion. So player three has no power. A player who has no power is called a dummy.

    Example \(\PageIndex{3}\): Dictator, Veto Power, or Dummy?

    In the weighted voting system \([57: 23,21,16,12]\), are any of the players a dictator or a dummy or do any have veto power.


    Since no player has a weight higher than or the same as the quota, then there is no dictator. If players one and two join together, they can’t pass a motion without player three, so player three has veto power. Under the same logic, players one and two also have veto power. Player four cannot join with any players to pass a motion, so player four’s votes do not matter. Thus, player four is a dummy.

    Now that we have an understanding of some of the basic concepts, how do we quantify how much power each player has? There are two different methods. One is called the Banzhaf Power Index and the other is the Shapely-Shubik Power Index. We will look at each of these indices separately.

    Banzhaf Power Index

    A coalition is a set of players that join forces to vote together. If there are three players \(P_{1}\), \(P_{2}\), and \(P_{3}\) then the coalitions would be:\(\left\{P_{1}\right\},\left\{P_{2}\right\},\left\{P_{3}\right\},\left\{P_{1}, P_{2}\right\},\left\{P_{1}, P_{3}\right\},\left\{P_{2}, P_{3}\right\},\left\{P_{1}, P_{2}, P_{3}\right\}\).

    Not all of these coalitions are winning coalitions. To find out if a coalition is winning or not look at the sum of the weights in each coalition and then compare that sum to the quota. If the sum is the quota or more, then the coalition is a winning coalition.

    Example \(\PageIndex{4}\): Coalitions with Weights

    In the weighted voting system \([17: 12,7,3]\), the weight of each coalition and whether it wins or loses is in the table below.

    Table \(\PageIndex{1}\): Coalition Listing
    Coalition Weight Win or Lose?
    \(\left\{P_{1}\right\}\) 12 Lose
    \(\left\{P_{2}\right\}\) 7 Lose
    \(\left\{P_{3}\right\}\) 3 Lose
    \(\left\{P_{1}, P_{2}\right\}\) 19 Win
    \(\left\{P_{1}, P_{3}\right\}\) 15 Lose
    \(\left\{P_{2}, P_{3}\right\}\) 10 Lose
    \(\left\{P_{1}, P_{2}, P_{3}\right\}\) 22 Win

    In each of the winning coalitions you will notice that there may be a player or players that if they were to leave the coalition, the coalition would become a losing coalition. If there is such a player or players, they are known as the critical player(s) in that coalition.

    Example \(\PageIndex{5}\): Critical Players

    In the weighted voting system \([17: 12,7,3]\), determine which player(s) are critical player(s). Note that we have already determined which coalitions are winning coalitions for this weighted voting system in Example \(\PageIndex{4}\). Thus, when we continue on to determine the critical player(s), we only need to list the winning coalitions.

    Table \(\PageIndex{2}\): Winning Coalitions and Critical Players
    Coalition Weight Win or Lose? Critical Player
    \(\left\{P_{1}, P_{2}\right\}\) 19 Win P1, P2
    \(\left\{P_{1}, P_{2}, P_{3}\right\}\) 22 Win P1, P2

    Notice, player one and player two are both critical players two times and player three is never a critical player.

    Banzhaf Power Index

    The Banzhaf power index is one measure of the power of the players in a weighted voting system. In this index, a player’s power is determined by the ratio of the number of times that player is critical to the total number of times any and all players are critical.

    Definition: Banzhaf Power Index

    Banzhaf Power Index for Player

    \[p_i=\dfrac{B_i}{T} \nonumber \]

    where \(B_i\) is number of times player \(P_i\) is critical and \(T\) is total number of times all players are critical

    Example \(\PageIndex{6}\): Banzhaf Power Index

    In the weighted voting system \([17: 12,7,3]\), determine the Banzhaf power index for each player.


    Using Table \(\PageIndex{2}\), Player one is critical two times, Player two is critical two times, and Player three is never critical. So T = 4, B1 = 2, B2 = 2, and B3 = 0. Thus:

    • Banzhaf power index of P1 is a-4rIMUXQO-0v8imTJb60b6Vw93cHlVyJ1g2xdcZkrUJbxpP1nEsj-jRqPfA_gJ_o-5aIVk1J7_1I21lJa5bV6JRM4HJ-YFkTtMnAscagHU_WZOhpaqFyFT0JdUaFagqJ9DlUCQ = 0.5 = 50%
    • Banzhaf power index of P2 is VgsHpX-nx7cfUvqwZ94kDeL8MIWIp0qbUK0nglz4F4po0j0AQ2Kdrb26ngLElxXEjD5vUynl_jrLhyk-HVCnSkqvM2h2K83s5HdkXSfe9aGBRttcvlBII6u9BEZ6ttPdyGkde6o= 0.5 = 50%
    • Banzhaf power index of P3 is vqL-wB7AIAmw4Ah9wjbrUbTOBwi51ToTnEvvIeb7L25USitvhB97mVGTXWNKIcSgw1N6LGTmYVxiIQ1uxJcri7k92cikyID-OzXnxPu-ZVr73qmXsBKIt07Ki87I3L_hhscdVb8 = 0%

    So players one and two each have 50% of the power. This means that they have equal power, even though player one has five more votes than player two. Also, player three has 0% of the power and so player three is a dummy.

    How many coalitions are there? From the last few examples, we know that if there are three players in a weighted voting system, then there are seven possible coalitions. How about when there are four players?

    Table \(\PageIndex{3}\): Coalitions with Four Players
    1 Player 2 Players 3 Players 4 Players
    \(\left\{P_{1}\right\},\left\{P_{2}\right\},\left\{P_{3}\right\},\left\{P_{4}\right\}\) xkLT9LBwhMIXDHh0JwVddx3N9WIvPBaSxG8cgjsdDWOJlK2nrwCMvPXr7sR5dmsRjyJEvOgQmHtS8dvuAJklJaIAozRDQmxGxNXYLJQPbLc4-T7Twxnk4dtLeSH_S6UMC0WyR30 k8ZEQWKObTt2eXv01uzkHxHYF196s1AHR7NT-a8YEyXFQEVHUWAMpyt6GDyKJRLlqWpcVf3YdLZim_lbHSIKp8al5yv2QkH4S-JcBpYZ9FiQfHKBCzeeCZJW7eqLRt3oMEbucco \(\left\{P_{1}, P_{2}, P_{3}, P_{4}\right\}\)

    So when there are four players, it turns out that there are 15 coalitions. When there are five players, there are 31 coalitions (there are too many to list, so take my word for it). It doesn’t look like there is a pattern to the number of coalitions, until you realize that 7, 15, and 31 are all one less than a power of two. In fact, seven is one less than qr5Lmg4aopbNvemWzfxws5RYsgrAVXIyP0dU6yXWjBBqS8dXE2zQsolw3AmE_yBwJyxbf8dW1wyNEcHI8376uO0tpExpp8JSPmQPUlJ0Ew_SeMsGJqPsG4slM2j1rRYliWLPMP8, 15 is one less than 39WVmlC9itTrcclSoMS7j-U1ktpt-ZGwVgUd18tHfIUJXlE2O5Pkg-IWuezdXgZCfDq8bdqvbkD_PH8JHCN8dNmzbdDG0gGIy37CEUVUNfr764Ua_cCa5_nAd8GwPiQdn5vHb1s, and 31 is one less than PHKpCGaLLbueyv8DJO-KzFrWJabFjyM9KRp3d3Fuf8Jk5BZmQiKfdkvdvgA0NhJS4kw-ZRDgPXZImPI2gbhuXbQ3Onf07mJy_YfpAsc68FMdLNId-eVPoMqUCIk1cegRuJoNVdw. So it appears that the number of coalitions for N players is rtlxaXXygUN9wQGnC7z3l3nC-_Lndf6mAzNsll5b6Oq3cdsJxV3_kWtVKbzv8PDsYLphj_LfHl6E14KxkQonFs5PEGR4LUq1HfrD-tX6A5Dx6zvicgm0CeoiOArKI6LkmIU9eNY.

    Example \(\PageIndex{7}\): Banzhaf Power Index

    Example \(\PageIndex{1}\) had the weighted voting system of \([58: 30,25,22,14,9]\). Find the Banzhaf power index for each player.


    Since there are five players, there are 31 coalitions. This is too many to write out, but if we are careful, we can just write out the winning coalitions. No player can win alone, so we can ignore all of the coalitions with one player. Also, no two-player coalition can win either. So we can start with the three player coalitions.

    Table \(\PageIndex{4}\): Winning Coalitions and Critical Players
    Winning Coalition Critical Player
    -HeXYgyfr5amCh4m7o_Wlh19H_8ODdYuGBxKpLhcSD1eZ2SzdmFDPzjG6K9ccdCVOQXSqZUeTyW3Ho5dGJE2ZP5NViHCpxBKdRKr-Bbq62FcZF3NVS9R5t6gjXw3d3NupAegLrE HTolUwt0W12BDAJ5AyqiGlZBIPFia8ixpjysBwKaoX2BiqP6kk-GA8W0HVeVAsZYeeCDluWzhTU9QpGN_gpGk1PHjlXVAoORFJr7CcCri1Fo2tIvaGanxOnCE-QuM6GYYD3UnU8
    F1JeHCA-HUksx5TFTTnC5BOKFeqRC2JbLY2PTZJxdeQ00ryY7pQSll3gtUyDvqrWbKIe8cok3Z-zPOjdwPErATjltHRLK4qRZABZCN-J5Q8LnxukoUEok7GmywyZumEC_zco6nU gkRorYpS-5LLIeBisfk6k8oDHF619AIGffzw7cj2CWHr3zTAelwJvB2YVWzgBnoMGaDbk28HkzEGoLWynddGGTJvDZXtRZbm_-_5iaKSa_4M0VZ4_lfbn4yIZEsxgUOi2aATVXk
    IuPCA8mPtNz2gIgf65ShncliAUg9R7VZw47SQxoNVHip4MaBp3LbD5AhfuniEZTi8bSB7SKKPXJUzUb3mgVKkNx2s0siwDhKJb308tI6SiHhVxuNLcvyrw-t125qE4r6P6I1Fys _QJ6pFW8FfwDfoOBcHl_Wyrc29ALeGevl9JZ2CbIMhASKUl9cY2hRHAQkB4HdboFAuLU6Cj7JWfSbVhk9xn0nkZvlLo4c8LOjrDcZCxrhXPzXCqvd_9iDGmkpIjFJdm_GCXMawc
    duy_qtVASdESqqJOJe3i3ApTby_lAGwCpbgV0KoVvHCZIh36-cTjqH0NJvtCZIbisSfaP6zNMk9YVn0x1p15uWVXjEmdTfHBvguCVDw1Hv1nSJs02C_VaixeL_LpqGC5FZaihWA LjMe7-KrGY-iYPN8EzTUmUdFWjPzPb5o8XF-3QSs0MRKNrbiNefd1RKYUSpntQD3vqiIFPj4mpJLkTdr-D93KaeMYz805PemL6kRgZmUvOv8bQ1GVKG5XeyGdCdPkW7UdcTkSes
    6huQHTPV1ryIcQmgQYNREEvhW4z8dCRYClMAcKQAJZooFdmjot3SaOSefZSbEUnJ4Jr3lH1J2pyeGFl4gUKAW22J6zhUyIsddSdrhExOvccT8Qfr7JTG83haP1QBRqTgf22oMpE wJB8J9SihfekUm5o5leJ8b8BiqKW2H5JSX9-gANu1WLUphPkA_tR5BT3syOt9K8SBX7U1ugHkIjTockJP18CvuU6Cv_93BOcXBsfCKz0ZWPUoX1LbIHVVEzgjTM2qu4l6-kIpWU
    J4sWdLpuR8cydEWLHr8ZLIJzcrqlyCvao-6tTrkEdic3wjMMmOD5v2EZdPJv7kQh_CIPk9_9s58m_Ah-SsNgsYAYmP9OG0Yv6A9d9aawlXoLYoM4PM4NzXpBVEfMq8wtY91ygaA 8qbHuGv4QbbTyFABpH3kt8PNo73XzwVQPXnE0FXEa6x8Jetp5kI3ep6LVVBwaZ-W5N_gsiak7NJ-5O1bkWPEiNDbXQOiNLW34xKPI3htY6egNilN7W9_9e4GxUsxtXF7FkCKSOc
    TtSJ3W79u8x_Jh-99qqTqTQObxttmBgRvk_Rfhwx57wOdQmGFQYVM5c8x1B9iXWxbE8XadUYzjtJ9pg0dmG9vgsKwLcRVfY_KyzjUnvwSyQ0boDrLcvlnZPmQcs-AT7OCuYYsJ4 rdr5QGVEZtc_9haT86bcaYI8YFSW8GStyParyKB9ZcaTEADjRUOd1aDi0Av67lW45umh4vuC5AKzGQs5YnzMM_lr4YlB68Plg_dg2DDWB5_w2mothxXkgwc0EFNuiaxqI9QxW-U
    7z8peHi0yxNraN5-wYViKED8RchP6fODWxI3_2XrXUCMACfVwlxgGh--C4bn6RMRyZWwHg_ui1tbbcsTdcPF2QgMnKwg39KxX5s2wzRA7c8jzudm5nwYBLWEP8r3DBC8JrwotPI Ixcp18WzO5MHjHzgIqzV4EnkNMM3nwGPUITQVaKAdorHOuhewp2MeCpKqKYkGdXWh8J93XgF5mO9wjOqXPV5UQ4A-I9pBZDpbiz9sW5uMpF5CVFmbLVahNc8DWYzxYEiQS8TuHg
    Mua8cBuJV8FEU8mvkOccvEVouHDtUeqoyWa7Seb3c9rURyAtq7KrBN4mo1etj3GHP5iEILc7HZEDGMPus1UxajxNTfMD9_Gk02sQAraGEw51g6gsCHjQWmVwmXnicVU5aV8-SfI UVBo4X3A3tMy34hSQEsOUpXkuWfDPPZQS6iHPwARkM1hYItsfYNZlXvCasg7AhoTuLLkwx6FFuP92HjU5ghUdoQl5bue8e9poYvPTpRihx6Eq7mYll-gHSKGR1ooUBUXWT0a3oM
    UdBLCclPdXaFFj_mf-5INZDf5yK3TCdTwXtD1if4kddnhJkGzXOK4VW_6Q9TDsel_fvbPt6aoSOTR4DSRrG0zAEEuq_JqS8R7Rp09JeQ1d1U81tvznfej04V9zA-Q6_8UbDlUPg 8a-qNP5kf1lajYRIZFc_EPGCWRzuMoTEnUjdPcA6HQs9nKGMDrd2Z_tGExIbQ27rPYWGdh8_A-8H-DswPOaHQzG5nHpSgjNiCmbOXNfcLqXDuqhP-lAGnt3FfTJcPzaT8HdopH0

    So player one is critical eight times, player two is critical six times, player three is critical six times, player four is critical four times, and player five is critical two times. Thus, the total number of times any player is critical is T = 26.

    • Banzhaf power index for P1 = 67G5XTHwdoy6LefjwVqDy7i9YFsBaWN-sEKPsu2uiwtZwk4N8C_8PP4CZkZmZR4ud8Py2dKxu3-ZNYa1RfqhbkMWc7iNUhrZPiGaMnntNCMzlIeprtquXDVTu9zeRf9fGr7HpBM = 0.308 = 30.8%
    • Banzhaf power index for P2 = h13d10srXnDhKU7nNiCFz9CrDr0liI80bpCr8-gEPtnkcsZYKwCAenN5Do1oNeOF26TQ-pryDnfb0tifoHizrjeKodD0f41FeoDOoVuWpX9UB7ZpLXjP11kvTPBx61pSm0zvlIE = 0.231 = 23.1%
    • Banzhaf power index for P3 = XHaiV7h6tmESsZZda7hzoPk4681lGJ2c6WxPgT_QkSxuKuDrH3jkOtv0R2mGBqy-ORxkI6BNgtOHCB4dxvX2DgEYXHSVNb471t4zA9UBdB2Cb45l54tWuEXANKUxT3uKGilHq-Y = 0.231 = 23.1%
    • Banzhaf power index for P4 = qxMpKx9nM_QJuC0jwq8ctgesz8eCGzpMa9bYvekb3i5pMunmGF9lTWF7kw9qnTgSH4pXdPoz_DxJrzxatUd_o3bcTLpvBnFcWkY8DQmSsLEKdCdbuLWyxwIy68YO_wRq3Rp8a80 = 0.154 = 15.4%
    • Banzhaf power index for P5 = _iQ1g6eTtAaRcQAMzFaSqhRECpTqtNJE6vrnUpuQGBUsEMjGU_qK3dSfv0y7h_APu37X9rCXF_GIHDescWK7YksLbXaJIZMSirNEwL6QMnysyS1KMnOW-PK0tk2wkKu8p-OQ9Qw = 0.077 = 7.7%

    Every player has some power. Player one has the most power with 30.8% of the power. No one has veto power, since no player is in every winning coalition.

    Shapely-Shubik Power Index

    Shapely-Shubik takes a different approach to calculating the power. Instead of just looking at which players can form coalitions, Shapely-Shubik decided that all players form a coalition together, but the order that players join a coalition is important. This is called a sequential coalition. Instead of looking at a player leaving a coalition, this method examines what happens when a player joins a coalition. If when a player joins the coalition, the coalition changes from a losing to a winning coalition, then that player is known as a pivotal player. Now we count up how many times each player is pivotal, and then divide by the number of sequential coalitions. Note, that in reality when coalitions are formed for passing a motion, not all players will join the coalition. The sequential coalition is used only to figure out the power each player possess.

    As an example, suppose you have the weighted voting system of OaGKRHWv5QWiwtsOKNTgMSMlPaOLTZ6tQQs1xStHVMwdBhsfNfU7iGdfRH5OgYuk4qRbM6Z4uTAU_124qCBqcNsE-_YHji-CGz0gvTRXfGJKNUkH5auEm1HQ0TOkU6XH5ZZ5Mq0. One of the sequential coalitions is RY5lF4d5q1xviyQEyA3Mser547mCwZqyJjfyNMIpdixZIbtzrxStfYeWLOlid8kdxC_ofBKmqp4m8Jhl4mlS8VQJrxppT4vSHf1sR2GneeVJfPzj5ZCMqPTfqcl_QZQGrcJzkro which means that P1 joins the coalition first, followed by P2 joining the coalition, and finally, P3 joins the coalition. When player one joins the coalition, the coalition is a losing coalition with only 12 votes. Then, when player two joins, the coalition now has enough votes to win (12 + 7 = 19 votes). Player three joining doesn’t change the coalition’s winning status so it is irrelevant. Thus, player two is the pivotal player for this coalition. Another sequential coalition ismW9AfPM_mIpR1PfkECdaFMOfar0fWENx2FxuguJVP49r1W_b3h74YW2SmveyfloIVW_LVcJWAkrPt6FEVR_Vu7wGl2EGVhuzBdO60nDTKscvGGDvWx-rrfOaXz_OE4GIf2ZXM_c. When player one joins the coalition, the coalition is a losing coalition with only 12 votes. Then player three joins but the coalition is still a losing coalition with only 15 votes. Then player two joins and the coalition is now a winning coalition with 22 votes. So player two is the pivotal player for this coalition as well.

    How many sequential coalitions are there for N players? Let’s look at three players first. The sequential coalitions for three players (P1, P2, P3) are: YIlhFMMcQyq_8azvbByzaFt_6-cHjoR7XHc4mLTjsGIuozJZYNcSQ6h7W65x-QmetuvrawWjgdKWMidP46aTQCQqeDQBx2t91InH8HSFQXSIK_L9hLcBPd314ooCuJ4LIKwom3c.

    Note: The difference in notation: We use 6bwuP5gPgeJr4A0O6NtrhTvZicBbi9wv_rloHknMr8myYqZeUMkz5lVVYtSWdpxiEGdnWCl0XhUB34A0QQqt1hIYuOO-FW8ufZxdAP73XLEa3snQMDt098Zd9z-ceXiyVoK9ZAs for coalitions and baKS3r9Lsdi0ewH4pc4T39OujN-5Etj2qnKH49wuGrUiE1YJ8wsWt5S3xS4WkMqf0heR5txI7bJP5aa0DlLeukOGiInZy27QUYT9-uD-eOQ8raRILXz3l5LTsp5xwEM8pbeF_qE sequential coalitions.

    So there are six sequential coalitions for three players. Can we come up with a mathematical formula for the number of sequential coalitions? For the first player in the sequential coalition, there are 3 players to choose from. Once you choose one for the first spot, then there are only 2 players to choose from for the second spot. The third spot will only have one player to put in that spot. Notice, 3*2*1 = 6. It looks like if you have N players, then you can find the number of sequential coalitions by multiplying hnbZWvKbOuYf5rj5bc-GOKwpIz-PDN3xSpI6Qe--6C100cYLLPmhIjMyk_F56AZ4v3N_6HrxqFEf5ThCfJ5dXJ8iR_MDdeFnHkxykc1Pc8IUnhPoJGBss-inc-7D8aQSLhHyTcY. This expression is called a N factorial, and is denoted by N!.

    Most calculators have a factorial button. The process for finding a factorial on the TI-83/84 is demonstrated in the following example.

    Example \(\PageIndex{8}\): Finding a Factorial on the TI-83/84 Calculator

    Find 5! on the TI-83/84 Calculator.

    First, note that VOdGoRTPr7mLbcRxBVeDhXy7G5h4D-_3PjkopfyY4lIuTds_vQw6esbsHBieNeCTAY1HBeJpADaVK3Lb54Suor9Obfcq1GtKl3TDG0r1JYXC5TDcnKLZZtWdr0ddkDAULW6ZooI, which is easy to do without the special button on the calculator, be we will use it anyway. First, input the number five on the home screen of the calculator.

    Figure \(\PageIndex{5}\): Five Entered on the Home Screen

    Then press the MATH button. You will see the following:

    Figure \(\PageIndex{6}\): MATH Menu

    Now press the right arrow key to move over to the abbreviation PRB, which stands for probability.

    Figure \(\PageIndex{7}\): PRB Menu

    Number 4:! is the factorial button. Either arrow down to the number four and press ENTER, or just press the four button. This will put the ! next to your five on the home screen.

    Figure \(\PageIndex{8}\): 5! on the Home Screen

    Now press ENTER and you will see the result.

    Figure \(\PageIndex{9}\): Answer to 5!

    Notice that 5! is a very large number. So if you have 5 players in the weighted voting system, you will need to list 120 sequential coalitions. This is quite large, so most calculations using the Shapely-Shubik power index are done with a computer.

    Now we have the concepts for calculating the Shapely-Shubik power index.

    Shapely-Shubik Power Index for Player Pi bUw1APikOEBmaME7_a4dq8MbzaF3PKkMlOCUrAQfSzDz4u8cv_7HGVtIdOd4Kv9DOFg5W7UEcs90RnmO41UCp8RfuybVHSWq7iALzQeDmH951_Yn7I4HsBcKTWqDaBSJVUO3m18

    where OJeagbrznr9TAc-AyDqeRVbEAEr3Pxzrp8jtawrA1JLWlceBts8l0KIasBduOjGawSAJLRCPS8mEl9E_53YUTPMA9HRyVavwm0oBHh-QjOMhlJWJYjorv_GnxttXszdeAl0Omng is how often the player is pivotal

    N is the number of players and N! is the number of sequential coalitions

    Example \(\PageIndex{9}\): Shapely-Shubik Power Index

    In the weighted voting system \([17: 12,7,3]\), determine the Shapely-Shubik power index for each player.


    First list every sequential coalition. Then determine which player is pivotal in each sequential coalition. There are 3! = 6 sequential coalitions.

    Table \(\PageIndex{10}\): Sequential Coalitions and Pivotal Players
    Sequential coalition Pivotal player
    kYzd0MjDnHPfkuGApY90KzO9eC3FbKAu0RxXnXAW7D7sHWd2CIksX78XhfAI6Jc8xohxpblmwA39kukw1YA2Y7Gqq9PQ9-ruQCctt_2R6sbiVYChk6X1z-QNb0TdmfLOriKDulk P2
    VQdNABcFgBdO6pb6VxhgQZNrrRA5zHvWJKGo8vTcmIVsokzo9ZpafImlC7MG2_npBQWOHgpPUlfklMvX2wNYeZuwHJFvu6BPYXF-ji9xHOikSMN5EjTTGjcAnFeZsRXY1c7KrMA P2
    uQPolAZh2TS1I6_yRThhqEVeQHJoJ0RpI69X096SGkFsXjnUDaQuZqIOcgXUbE83C5zNdAuFNp9D6DKzbmGJUzdx4XpLiW-oat-17yU4X4RwOGryjE2dr_7kJ3Z6spo8g5rsUsk P1
    rBJcO_ipDwHCEJaaCqm2sbdu0e8YFYYGBhIT_SkvHipOqSUAMHic60q8tDs6rRO0RNJQkqfhxOA_tp80F5KXntlOUAqI0umOQmgxsmJnGTeq_AEue3-6j6to0J5Od7qt1-YQlFA P1
    ZMKU40ptF3B4fIKopXTHo92RovItqjb143tXk2DmRh5j0x-DmWQheICuAVqoKb2EV2fxNeLhDj76CjaA3KEeI4MskwnmtKVIeD3vZa3GzdpPSXKsDyJSfoM1P9KJCaipkmewOiU P2
    Hjg6XqdiSFH-yxclWwNM73viMw9_zFRfSesKxffUaj-QT_JK_PX37EFC6O4I7iRd3GbXmnBJtqbFpQHSYKM0E4NuUvBd0qd5qm5EBdupbEh8D7X4AQc3M4KLtdy8oaYSFu2d1XY P1

    So, O9ngQwcq0OigCjJEIrBGQdFiWICaxuFVq0zEYK4H4LXwD1NxvEvbhWp2pqQ0cEnJ22alj_VhH52iVzj0T2MlWEv82Ozm9eHwRLg7u1TRuoePKXSXWr2K337o_vhTCUjHP_uLIbY, tMmHtbDMXFkjRx1bYw74iaSMOtD865algrt6QPnRajs6rmYuvO1ztbHwLyMyqea46CZv2VQS4HZBqCUxyac8-HDAVuxfGZX0vDL3yBzOQGhoyAd_3oQA5N99mPUGclQx_9AeppU, and HyPj25ET6Fhkkrd9flfkbBtsrJUdt6XXGOi-yXyBok8tWcvgKQ6dFVjr-XFcx--KRHMZVD5Vl_QL_E2yXjkpvvI_74gLzoLaqs-Un1-vlaAFIHoGFJZJPmwuIYCHZNzHfPCkpSE.

    Shapely-Shubik power index for P1 TvG_IhvSTRfGlcw1L1x3hfv5wy-yaTa-ELz8a9sJbew7oot1TEdpIcAHrMnW11QhcLIG74VMrDMhVXrAtenZRLOgpb68H1PbvdjlppIeVFIpj9Wk7IB74WgQoWcY2rX8hQ8wuIc = 0.5 = 50%

    Shapely-Shubik power index for P2 VAr71YFI8Isud816dZ0aOxXcIIQeqvOgjLQiMRaqcwewqHnOD7_nM_dycJ5YeARQ35wwouWcfnTM4uruLZWkcSjz01MD5tZaMwPNKe7mON5rzWDim3HZcdj_HH2QA0bAKJ_49jo = 0.5 = 50%

    Shapely-Shubik power index for P3 m3vppMGVKdvHpppPT4DhSfeajgTCTMYuKZoQ-G1f9rGuY8_fXMICNL31qNDLNVS8ybZiqjok5a6x8jHFC4be4snrhl0JLTqbIeh3cBnvJHW9d_dHTX9kP5YfGniYM0ZhW1qCH0c = 0%

    This is the same answer as the Banzhaf power index. The two methods will not usually produce the same exact answer, but their answers will be close to the same value. Notice that player three is a dummy using both indices.

    Example \(\PageIndex{10}\): Calculating the Power

    For the voting system pshoFTnKjSO67tGYUNG_GvlZDT8-QXhYZdFWitSAYAMC2ytn71u2CInnqGvDz3t6tf6IzC_EgHq0StnggrNft_pJzqkCFxkWjml6roeolZ2vil7u_M5Ef1FdG0pXXeI861Q5z9E, find:

    1. The Banzhaf power index for each player

    The first thing to do is list all of the coalitions and determine which ones are winning and which ones are losing. Then determine the critical player(s) in each winning coalition.

    Table \(\PageIndex{11}\) Coalitions and Critical Players
    Coalition Weight Win or Lose? Critical Player
    JJ-LCCOEUxlGPj8eIOteHHDu6msOeeh5wXKIzzNnGNUgL0L1jQTVXvSYsSxhK01sozLux1N9PqhGNHLo5SoC-WCZeNpT6UZBYkEWXoxcodA_cX6TwNRzD87GYxzWAVowAJyiyqU 6 Lose
    YZcH3D2xcbmelrdX2GZkHrFAORd6DAkNjtUhPumzTleA3mMlY2wx4wyXuFHLFIVgYkONHMK2PwfKXvjZYJUcwakI1QmRCw-DaaNeH4R3aYqLqocpyy9d3aTxmYF5H23Y1lAVhKI 4 Lose
    v1CNX5HfV2_BNjbZ9yHyigjIgkJPCszXDUtUSxMqVDrb4scFPwJ4J0eSatxpY5MWR4Z6nRikapWbtlulyaL65Ac9YxW54ZdtMohTSrKNRBptZQqlH2tWoyzi2OD_hzpwL3k2gog 2 Lose
    gV_BlQjFlWzkvPc3YP_psy3drTzFywI8NqnPUBXlE3IOyijkqDGS7adnqR0Nifi_ymzIi_E7F3ytElIIYU7oDYsPTAO0U5ee3MFughsEx9ESUrOzccWZNyHaDVl2LeVkVhTBCOQ 10 Win P1, P2
    qQhMVRyxnvcjSXuiFhHapQE2AHSQvy5shgIL20OsHCfCEfwN9eJRd408ofJPhtmr7lILta9gbWVzXk19uu4Cdkr5x-NE621fRHqaYlHOt7Zuum-aN9mQf4W8gsHY4rpAQNLB18w 8 Win P1, P3
    nEdikPLPeCv8hCgg_uj4HGUSGdv0FamyrrpxjqI-_V6fkpWJk9fqzHFFxmOBjaAE6ejUI6HhaBt-qDzL7oiraA-yNCAozvQ2ImcQznknof7jqaVz_WA05tUTsE87Fu7iB9pltos 6 Lose
    rjOw-OBLegk2VU0Jw6DC6dOvZwMIyqfRCT2eXeQwnWGodopSTFDX67SWS0CNkYjQOlNr1NryhiM4Qyayp6bzvjumPQPN_LqmY20OaIdz1BsQOMkiP3HBo3evcM3oE-4ndOtazqw 12 Win P1

    So, fkpX6vVBcaq9qr31IhsIZQnu8b_UHvdhea1JhKuaDfHt1T4heFaGsid0ZwtxLBgFNTWyuNVvlAV_0cRriOjPzlfTX8g-buU9e4sr9oVzk1LWusvkQe3L6nJxG2LqQwnOyfgDKWU

    Banzhaf power index of P1 VGIwqgZD95EvMUrSU_wdTk8LoZ6MOndFVxQ6moF4_BZCH29MdkyhFMOwc1Psko-zDFozDs0pFwf0EX-rcmCw4k5YBTMPGcz40ZLzcg9A5oYUsARIWMp95idyzGcGa_7tQVFYh2I = 0.6 = 60%

    Banzhaf power index of P2 OlfZgVFv5dek8sYPxiG_Zois8bJMUR2or76fgYQ4oGD7Q72p-dkwX3LT1t4ekpM9qkMWohfazMipbYe4G8aztwKuZAsRlvaDgPcKSBdcp8Tv0eIOWiubsg1RIeTvw4brfTkxG2A = 0.2 = 20%

    Banzhaf power index of P3 jmaGLNEVo915uD6llGZJlfFhUOuNmk2yXZg7Kv4M9Y7eGuggWQB-EAD1fBigRMhWkG1erCT5c-6n1vkF-A0jxl-EmCeQ5w7zqoRt9d10udV8-6VIW9KMtYdAk81A_tWKOdWDuFI = 0.2 = 20%

    1. The Shapely-Shubik power index for each player

    The first thing to do is list all of the sequential coalitions, and then determine the pivotal player in each sequential coalition.

    Table \(\PageIndex{12}\): Sequential Coalitions and Pivotal Players
    Sequential Coalition Pivotal Player
    cUIibZpZ-cXdzd_70Usl9ymfDwhXSGjcFeNMkSqb0zy5fWYL5oR1GowNeVe5IxknlOAZC5irp6rtYh4iYZV-GO9jn1e7oD35BJfuCDXkB-n-jECVzEwULnxFmZuZC2ZGY9L9gDo P2
    LSZfq7LcZoPx7ctpjJNOhDlK_W1MWa4EY9YOd4OSTLfCZ5kt4RNdP8cx39SKRJyuWA3bOPmhped72mivTRQyVPfY8gXTieiPOoBMDmG5nScc3I6-LcOzvlwApe1MhiFYSyYQZx4 P3
    nCwnh_MVOTe_VEeaPUe5cq3HB0sZ7X4FeFkapCqoXeklMuvJF2iSvsgatn-HLJjEGgdEINBDATyqkD37oDELdGXOgl7y4bsSm4aGQWorOnufsUFqMRDlNPvdvyJtGUAqgGrZ4LU P1
    aSPccDSWDJhC_ovVfljGnKfYDxAm6bHDKkAgnde2WFsSaxpqdD7hIdg64xw9GUKqD7yxGU7kZXJBm1OqWBodbkkdHZtN9xO-huQ9JKIl1XRv8ybjwc2cXah4w6BkMpjgWg9Herg P1
    YFO9HNagKTenB4k2PLZ9BVFNmciGWQpTGoG3e08zWp7JepMG4CMIyckXUuyCnJZWRXloKdxK7cS8Km0YAtH4EzYWFHHhIWjmJUGTpJZl5mWT_uSX2ElfOVjZV3Uwlb3cJwVB3jo P1
    Qs6B2xELnkr8Y3MuZdefNQ4wvSYzsM-7CDOO7govN2044yiIL0aBAes-Isa_3Ic6csfIfRiWv-vfl_q5MqK1DqfapkBIzUaugQLFWpJ5P9-q8M86QlaWO38jSBBHcSRCXJ_kikU P1

    So Cisnv_F3mbiXUs69d_D1fJZ2hdHWw1umsAvl1rAlYDq2904smK-84UXenps4yy-_zPgWf3h5q6RxfutawIey_03l8ss25vBcJbGqt_Y6t914qoI5r2h5laScxNFe0-dV3IB550Y

    Shapely-Shubik power index of P1 obwwEd4ommYCFF1ft8xZ5IBrYnbntRb76Bv2ctHzhi62AsFtguPyfl7kADKonbTLc8RNxkrGhyI5LpVqiYSoDrFBnK5X9B-iM-Z0tDns8dfFR4_kpQIKjOOmpnNMBn4rdMOWrQw = 0.667 = 66.7%

    Shapely-Shubik power index of P2 tv7OGQKw_-QBMGl01ywdNH0dLHiPAxhr_796zicdqvYDYQMIUj3-GsskMseDIv34dsH-RSFThUt8BSzWZi7JyDV0ro3SsRinNmWEZlp_u-bT7f51GXN0f0xKvbaGfu6wkBoKlrc = 0.167 = 16.7%

    Shapely-Shubik power index of P3 EzhF6HZdfwA6S-KpJ0tPNORPcGlex-EKGY5ywQEUYv3vu1DAk-qs-evoo0Ehn5NZZzctf_2tg4PKldP4BNpcz2lX0frUUhpTHv-UlEEFqBddjzPFWrL0DC4sd2yvLKoiReg3CiU = 0.167 = 16.7%

    Notice the two indices give slightly different results for the power distribution, but they are close to the same values.

    This page titled 7.2: Weighted Voting is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Maxie Inigo, Jennifer Jameson, Kathryn Kozak, Maya Lanzetta, & Kim Sonier via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.