Abstract. Consider an election where N seats are distributed among parties with proportions p1, . . . , pm of the votes. We study, for the common divisor and quota methods, the asymptotic distribution, and in particular the mean, of the seat excess of a party, i.e. the difference between the number of seats given to the party and the (real) number N pi that yields exact proportionality. Our approach is to keep p1, . . . , pm fixed and let N → ∞, with N random in a suitable way.In particular, we give formulas showing the bias favouring large or small parties for the different election methods.