Estimating party influence on roll call voting: Regression coefficients versus classification success

Contrary to the claims of McCarty, Poole, and Rosenthal, our method does no t estimate the ideal points of moderates significantly less accurately than the ideal points of extremists. This is true for at least two reasons: (1) there is significant randomness in voting; as a consequence, on a lopsided vote Moderates often vote with the extremists; and (2) our data set includ es some roll calls that require a supermajority for passage; for these we d efine a 50%-50% roll call as lopsided. We also show that the classification -success method of McCarty, Poole, and Rosenthal severely, understates the presence of party influence. Furthermore, we show that a proper interpretat ion of some of their results reveals a significant amount of party influenc e in Congress.