COMPARING SURVEY DATA ON FUNCTIONAL DISABILITY - THE IMPACT OF SOME METHODOLOGICAL DIFFERENCES

Study objective - To examine the impact of some differences in survey methodology on the prevalence of functional disability in population b ased surveys of the elderly. Design and methods - Nine surveys of Dutc h people aged 55 years and older were compared to investigate the diff erences in the methods of data collection (proxy questioning, yes/no; interview versus self administered questionnaire) and construction of the questionnaire (wording of introductory text, activities, and respo nse categories). The effect of these differences on prevalences in thr ee domains of functional disability - activities of daily living, mobi lity, and communication - were studied. Both univariate analyses and m ultivariate logistic regression were used to quantify the methodologic al influences. Results - No effect of proxy questioning could be shown . Self administered questionnaires yielded higher prevalences of disab ility than interviewer administered questionnaire - in particular for mobility (odds ratio (OR) 1.4, 95% confidence interval (95% CI) 1.3, 1 .6) and communication (OR = 1.7, 95% CI 1.5, 1.9), resulting in preval ence differences of 9 and 11 percentage points respectively. Seemingly minor differences in the structure and wording of the questionnaires resulted in major differences (up to 15.6 percentage points) in preval ence estimates of functional disability. These differences were associ ated with the severity level of the disability indicated by the wordin g of the questions. Conclusions - Differences in survey methodology ha ve a substantial effect on the prevalence estimates of disability in t he elderly. These differences should be taken into account when making international comparisons and studying time trends based on survey da ta.