The Results Are Only as Good as the Sample: Assessing Three National Physician Sampling Frames

Databases of practicing physicians are important for studies that require sampling physicians or counting the physician population in a given area. However, little is known about how the three main sampling frames differ from each other. Our purpose was to compare the National Provider and Plan Enumeration System (NPPES), the American Medical Association Masterfile and the SK&A physician file. We randomly sampled 3000 physicians from the NPPES (500 in six specialties). We conducted two- and three-way comparisons across three databases to determine the extent to which they matched on address and specialty. In addition, we randomly selected 1200 physicians (200 per specialty) for telephone verification. One thousand, six hundred and fifty-five physicians (55 %) were found in all three data files. The SK&A data file had the highest rate of missing physicians when compared to the NPPES, and varied by specialty (50 % in radiology vs. 28 % in cardiology). NPPES and SK&A had the highest rates of matching mailing address information, while the AMA Masterfile had low rates compared with the NPPES. We were able to confirm 65 % of physicians' address information by phone. The NPPES and SK&A had similar rates of correct address information in phone verification (72-94 % and 79-92 %, respectively, across specialties), while the AMA Masterfile had significantly lower rates of correct address information across all specialties (32-54 % across specialties). None of the data files in this study were perfect; the fact that we were unable to reach one-third of our telephone verification sample is troubling. However, the study offers some encouragement for researchers conducting physician surveys. The NPPES and to a lesser extent, the SK&A file, appear to provide reasonably accurate, up-to-date address information for physicians billing public and provider insurers.