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Given a special case of WVG (Weighted Voting Game) of $a$ 1s and $b$ 2s and a quota q, $ [q:1,1,1,1..1,2,2,..2] $. I need help with calculating the Shapley value of a player with a weight of $2$ and a player with a weight of $1$ as a function of $a$,$b$ and $q$.

I know how to calculate the Shapely value in general, but I would like to get a simple closed form for the result. I tried to think about what happens if there are only players with weights of $1$ (simple scenario $ a=2k-1,b=0,q=k $ I think you will agree that $ \phi_{1}(v)=\frac{1}{a}=\frac{1}{2k-1} $. the simple calculation is $ \binom{2k-2}{k-1}(k-1)!(k-1)!=\frac{1}{2k-1}=\frac{1}{a} $ which is $ \frac{1}{2k-1} $. The calculation for a player of weight 1 (when there are 1s and 2s) would be a sum of choosing i players of weight 1 and $ \frac{q-i}{2} $ players of weight 2. similar calculation for a player of weight 2.

I tagged binomial coefficients because I thought they could be useful for counting the occurrences.

Thanks in advance, Mati

also posted here: http://mathoverflow.net/questions/107507/calculating-the-shapely-value-in-a-weighted-voting-game

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Joriki, please refer to the edit in my question. Thanks. –  Mati Bot Sep 19 '12 at 14:06
    
Thanks for the quick response. I've removed my comments and downvote (and in fact upvoted). I doubt that there's a nice closed form, though. The result will depend on the quota if the quota is close to $0$ or $a+2b$, but for small $b$ there will be a plateau of intermediate quota where the result doesn't change with the quota. –  joriki Sep 19 '12 at 14:23

1 Answer 1

Note that the Shapley value for this game is the probability that a player is pivotal in a random ordering. Now the probability that a 1 player is pivotal is the probability that the sum is q-1 when she is reached. While for a 2 player it is the probability that it is q-1 or q-2. This should simplify the analysis, but I haven't worked out the details.

However a simple conclusion is that the Shapley value for 1 players is approximately 1/2 that for 2 players for reasonable size values and away from extremes, so the values are approximately 1/(a+2b) and 2/(a+2b) respectively. In particular this holds in the limit where a,b go to infinity and q=y(a+2b) for 05 and 2/a

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