Probabilistic graphs and power indices - An application to the Spanish parliament

In this paper a method is offered for calculating the power of political parties in the Spanish parliament. This is a modification of the Shapley-Shubik index which allows the ideological affinities between parties to be taken into account. It extends Myerson's model of games with restricted communication. A version of Hart and Kurz's model is also used to study the stability of government coalitions.