Measuring parliamentary deviation

Deviation between party seat shares and party expected power in parliamentary winning coalitions is measured with a new formula. In analogy with electoral deviation, in which proportions of votes and proportions of seats for each party are compared, proportions of seats and the proportions of influence in parliamentary majorities for each party are compared. A new parliamentary power index, which can be applied to different criteria in coalition formation, is used for this purpose. Formal trends of parliamentary disporportionality are presented. An empirical application for real elections in the period 1972–1993 in five European countries with parliamentary regimes shows a positive correlation between electoral deviation and parliamentary deviation.