In this paper, the authors present an objective system to evaluate interviewer performance for use in epidemiologic studies. With this quality control system, all study interviews are audio-taped, and a random sample of interviews are coded according to interviewer behaviors, such as whether the interviewer asked the questions exactly as written and used the probes appropriately. With the use of data obtained from a large case-control study of colon cancer, the authors observed that 94.2% of all questions were asked in the same manner by all interviewers and that 89.5% of all probing behaviors were appropriate. They show that questions that required additional interviewer behaviors were more likely to result in variation in response that can be attributed to the interviewer. These findings have implications for study design and interpretation of study results. From simulations, the authors have estimated the impact of uncorrected interviewer variability on study power and ability to detect disease associations. Uncorrected interviewer variability could decrease study power from 84% to 56%. From simulations, the authors observed that odds ratios could be biased downward from 1.8 to 1.3. These findings illustrate the importance of using a continuous quality control program in epidemiologic research.