Between August 1, 2005, and October 31, 2006, trained nurses and physicians reviewed the patient records during a 3-stage process. During the first review stage, 66 nurses reviewed the randomly selected patient records in a consecutive order (1 nurse per record) and determined whether any of the 18 triggers (clues) for a potential AE were present (eg, readmission or unexpected death). Patient records with 1 or more triggers were selected for further review (4317 patient records). Fifty-five physician reviewers from the medical specialties of internal medicine, surgery, neurology, and pediatrics participated in the second review stage. During the previous stage, nurses indicated which medical specialty would be most suitable for reviewing each particular record. Two physician reviewers from the indicated specialty reviewed each record independently and determined whether an AE had occurred. The determination of an AE was based on 3 criteria: (1) an unintended (physical or mental) injury that (2) resulted in prolongation of the hospital stay, temporary or permanent disability, or death and (3) was caused by health care management rather than the patient's disease.20 If an AE was identified, a variety of questions about the AE were asked, such as AE category (diagnostic, surgical, drug/fluid, medical procedure, other clinical management, discharge, and other), causes, consequences, preventability, and most responsible hospital department.