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

The objectives of this study were (1) to illustrate the statistical problems 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 dichotomous indicator variables for each health concept and show that these indicator variables facilitate comparisons between the diseased sample and the general population. Our "diseased" group consisted of 136 sequentially consenting patients referred to the syncope clinic for assessment and treatment. Participants completed the SF-36 questionnaire before undergoing diagnostic testing. General population norms for the SF-36 are available from the responses of 2474 participants in the National Survey of Functional Health Status, conducted in 1990 in the United States. Comparison of the SF-36 in a diseased sample with general population norms is difficult, owing to skewed and unusual distributions in both groups. In addition, making comparisons within age and gender strata is difficult if the within strata sample size is small. We propose a dichotomous indicator variable for each health concept that classifies an individual as having impaired health if he or she scored lower than the 25th percentile for the appropriate age and gender general 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 norm is the same for each of them. These indicator variables are age and gender adjusted, so that even if the sample did not have the age and gender distribution as the general population, comparisons can still be made with the value of 25.