Unanimous votes are the
  equivalent of test items that all subjects score “correct” (or
  all subjects scores “incorrect”); since there is no variation
  among the legislators/subjects, these votes/items provide no
  information as to latent traits (ideology, preferences, ability).  A
  reasonably large number of rollcalls in any contemporary U.S. Congress
  are unanimous.
Specific methods are provided for objects of class
  rollcall or matrix.