Teaching status is a critical variable in our analysis. The NIS classified patients as “teaching” if the admitting hospital had any type of educational program. For example, the NIS teaching variable would have erroneously classified surgical patients at teaching hospitals as teaching, regardless of whether their primary providers were residents (under the supervision of attending physicians) or attending physicians (with little or no resident involvement). To reduce this error, we first classified all patients in each hospital as internal medicine, general surgery, urology, or orthopedics patients on the basis of their principal diagnosis. We then classified patients as teaching if the hospital had a corresponding internal medicine, general surgery, urology, or orthopedics residency program (based on American Medical Association records). In addition, in hospitals with family practice programs, we classified internal medicine patients as teaching.