Roll Call Data and Ideal Points

We show that, in the absence of symmetry or other parametric restrictions on legislators’ utility functions, roll call voting records cannot be used to estimate legislators’ ideal points unless we complement these data with information on the location of the alternatives being voted upon by the legislature. Without such additional information, the roll-call data cannot differentiate between distinct, arbitrary, sets of ideal points for the legislators no matter how large the roll call record or how low the number of policy dimensions. On the other hand, when the location of voting alternatives is known, we derive simple testable restrictions on the location of legislators’ ideal points from the roll call data.