2005, Vol. 15, Issue 2, an item 3

MAZURKIEWICZ M., MERCIK W. J., Modified Shapley-Shubik power index for parliamentary coalitions

Looking at the process of coalition formation, especially in the parliament, one can find the following two ways of analysis: political party’s aspiration to have simple majority and proximity between members of joined parties in an ideological space. The assumption of parties' proximity is very reasonable when a power distribution is analyzed. Classical power analysis does not involve preferences of players (parties). Classical power indices are constructed under assumption of equally probability of occurrence for each coalition. The paper contains a proposition of relaxation of this assumption, based on extended Shapley-Shubik power index approach. Computation only based on observed statistical data (voting results) is obtainable with respect to this modification. An empirical example of Polish parliament was given. The analysis involves general parliamentary election in 2001.

Keywords: power index, ideological preferences, ideological distance.