Protective factors against adolescent problem substance use among regular tobacco and marijuana users: Susanne M. Stronski-Huwiler, MD, Marjorie Ireland, PhD, Michael Resnick, PhD, Pierre-André Michaud, MD, Francoise Narring, MD, MPH. Schools of Medicine and Public Health, University of Minnesota, Minneapolis, MN. Institut Universitaire de Medecine Sociale et Preventive, Lausanne, Switzerland (Swiss National Science Foundation)