[1] Univ Penn, Div Gen Internal Med, Philadelphia, PA 19104 USA
[2] Univ Penn, Leonard Davis Inst Hlth Econ, Philadelphia, PA 19104 USA
[3] Pharmacia & Upjohn Inc, Kalamazoo, MI 49001 USA
[4] Univ Virginia, Dept Neurosurg, Charlottesville, VA USA
[5] Univ Virginia, Virginia Neurol Inst, Charlottesville, VA USA
[6] Georgetown Univ, Med Ctr, Div Gen Internal Med, Clin Econ Res Unit, Washington, DC 20007 USA