Party power in the House of Councillors in Japan: An application of the nonsymmetric Shapley–Owen index

We evaluate the party power distribution in the Japanese House of Councillors using the nonsymmetric Shapley–Owen index. This index, a generalization of the well-known symmetric Shapley–Shubik power index, takes account of the parties' ideological differences. We first determine the parties' ideological positions by performing the factor analysis of the votes observed during the period of 1989–1992. The data show that the distribution of bills submitted to the House was very strongly biased; this contradicts random appearance of bills that the index basically assumes. Thus we evaluate the party power without supposing the random appearance of bills; we use just the observed data instead. In our analysis, the index thus calculated gives very convincing results.