1. Department of Statistics, University of Michigan, Ann Arbor, Michigan 48109;
2. Institute for Social Research, University of Michigan, Ann Arbor, Michigan 48106;
3. Treatment Research Center and Center for Studies of Addictions, Department of Psychiatry, University of Pennsylvania, Philadelphia, Pennsylvania 19104;
4. Philadelphia Veterans Administration Medical Center, Philadelphia, Pennsylvania 19104, and Treatment Research Center and Center for Studies of Addictions, Department of Psychiatry, University of Pennsylvania, Philadelphia, Pennsylvania 19104;
5. Department of Statistics, Institute for Social Research, and Department of Psychiatry, University of Michigan, Ann Arbor, Michigan 48109;