Understanding patient safety / Robert M. Wachter, MD, Professor and Chair, Department of Medicine, Holly Smith Professor in Science and Medicine, Marc and Lynee Benioff Endowed Chair in Hospital Medicine, University of California, San Francisco, San Francisco, California, Kiran Gupta, MD, MPH, Assistant Professor, Department of Medicine, University of California, San Francisco, Medical Director for Patient Safety, UCSF Medical Center, San Francisco, California.