Refusal and information bias associated with postal questionnaires and face-to-face interviews in very elderly subjects.

Our objectives were (1) to analyze the refusal bias associated with postal questionnaires and face-to-face interviews and (2) to compare responses elicited from a postal questionnaire with those provided by a face-to-face interview in subjects over 75 years of age. Our methods included a postal questionnaire sent to a representative sample of community-dwelling elderly individuals (n = 842). All subjects were also invited to participate in an in-home interview conducted by a trained nurse. One hundred and six subjects (12.6%) were nonrespondents to the postal questionnaire. These nonrespondents were more cognitively impaired, more disabled, and showed a higher 1-year mortality rate (13.2 versus 5.2%) than respondents. Late respondents who needed a reminder letter were similar to nonrespondents. One hundred and eighty-seven subjects (22.2%) refused to participate in the home interview. These nonparticipants were similar to the participants on age, sex, 1-year mortality rate, and responses given to all but two postal questions. Comparison of postal questionnaire responses and face-to-face responses revealed that agreement was substantial to moderate for factual information (kappa = 0.41 to 0.80) but poor for clinical information (depression, cognitive impairment, disability). A short index including age, sex, and six questions on disability was useful in identifying disabled subjects by a postal questionnaire. Our conclusion is that nonresponse bias is evidenced when postal questionnaires are utilized in the very elderly population and caution should be used when interpreting health data obtained by this technique. Refusal bias from face-to-face interviewing is less important in this population.