Statistical issues encountered in the comparison of health-related qualityof life in diseased patients to published general population norms: Problems and solutions

The objectives of this study were (1) to illustrate the statistical problem s encountered when comparing health-related quality of life (HRQL) measured by the Medical Outcome Study Short Form-36 (SF-36) in a diseased group to general population norms, and (2) to define age- and gender-standardized di chotomous indicator variables for each health concept and show that these i ndicator variables facilitate comparisons between the diseased sample and t he general population. Our "diseased" group consisted of 136 sequentially c onsenting patients referred to the syncope clinic for assessment and treatm ent. Participants completed the SF-36 questionnaire before undergoing diagn ostic testing. General population norms for the SF-36 are available from th e responses of 2474 participants in the National Survey of Functional Healt h Status, conducted in 1990 in the United Scares. Comparison of the SF-36 i n a diseased sample with general population norms is difficult, owing to sk ewed and unusual distributions in both groups. In addition, making comparis ons within age and gender strata is difficult if the within strata sample s ize is small. We propose a dichotomous indicator variable for each health c oncept that classifies an individual as having impaired health if he or she scored lower than the 25th percentile for the appropriate age and gender g eneral population strata. By definition, the prevalence of impaired health in the general population is 25% for all eight health concepts. Comparison between the eight health-concept variables is easy because the population n orm is the same for each of them. These indicator variables are age and gen der adjusted, so that even ii the sample did not have the age and gender di stribution as the general population, comparisons can still be made with th e value of 25. J CLIN EPIDEMIOL 52;5:405-412, 1999. (C) 1999 Elsevier Scien ce Inc.