1. Department of Statistics, University of Pennsylvania, Philadelphia, PA 19104, USA and Center for Pharmacoepidemiology Research and Training, University of Pennsylvania, Philadelphia, PA 19104, USA
2. Department of Statistics, University of Michigan, Ann Arbor, MI 48109, USA
3. Treatment Research Center and Center for Studies of Addictions, Department of Psychiatry, University of Pennsylvania, Philadelphia, PA 19104, USA
4. Institute for Social Research, University of Michigan, Ann Arbor, MI 48109, USA