How should we select health professionals for studies?

The investigation of healthcare professionals' practice patterns has increased sharply, in part driven by the development of practice guidelines; however, the optimal way to select providers is not known. We evaluated three distinct sources of physician specialists for completeness and potential biases. Professional society directories, which are frequently used to identify providers, provided biased populations. A national registry, the American Medical Association master file, produced the most comprehensive, least-biased single source.