  Roll call data are widely used to assess legislators' preferences and
ideology, as well as test theories of legislative behavior. In particular, roll
call data is often used to determine whether the revealed preferences of
legislators are affected by outside forces such as party pressure, minority
status or procedural rules. This paper describes a Bayesian hierarchical model
that extends existing spatial voting models to test sharp hypotheses about
differences in preferences the using posterior probabilities associated with
such hypotheses. We use our model to investigate the effect of the change of
party majority status during the 107th U.S. Senate on the revealed preferences
of senators. This analysis provides evidence that change in party affiliation
might affect the revealed preferences of legislators, but provides no evidence
about the effect of majority status on the revealed preferences of legislators.
