Interviewer variability - quality aspects in a case-control study.

Quality assurance and quality control are important for the reliability of case-control studies. Here we describe the procedures used in a previously published study, with emphasis on interviewer variability. To evaluate risk factors for acute pancreatitis, information including previous diagnoses and medication was collected from medical records and by telephone interviews from 462 cases and 1781 controls. Quality assurance procedures included education and training of interviewers and data validity checks. Quality control included a classification test, annual test interviews, expert case validation, and database validation. We found pronounced variations between interviewers. The maximal number of interviews per day varied from 3 to 9. The adjusted average (95% CI) number of diagnoses captured per interview of cases was 4.1 (3.8-4.3) and of controls 3.5 (3.4-3.7) (excluding one deviating interviewer). For drugs, the average (95% CI) number per interview was 3.9 (3.7-4.1) for cases and 3.3 (3.2-3.4) for controls (excluding one deviating interviewer). One of the fourteen interviewers deviated significantly from the others, and more so for controls than for cases. This interviewer's data ;were excluded. Nonetheless, data concerning controls more frequently needed correction and supplementation than for cases. Erroneous coding of diagnoses and medication was also more frequent among controls. Thus, a system for quality control of coding practices is crucial. Variability in interviewers' ability to ascertain information is a possible source of bias in interview-based case-control studies when "blinding" cannot be achieved.