Estimating a preference-based single index for the Impact of Weight on Quality of Life-Lite (IWQOL-Lite) instrument from the SF-6D.

OBJECTIVE To facilitate economic evaluations of interventions for treating obesity, we estimated a preference-based single index for the Impact of Weight on Quality of Life-Lite (IWQOL-lite) instrument by mapping it onto the SF-6D preference-based index. METHODS A heterogeneous sample of 1972 individuals, composed of community volunteers and participants in weight loss programs, clinical trials, and gastric bypass studies, completed the IWQOL-lite, an obesity-specific measure of health-related quality of life (HRQOL), and the SF-36, a generic measure of HRQOL converted into the preference-based SF-6D. Models of the relationship between the IWQOL-lite and SF-6D of increasing complexity were estimated by regression analyses. RESULTS The best fitting model for predicting SF-6D index scores entered levels for each item as independent variables (R2 =0.530 in the cross-validation sample, with a mean absolute error of 0.0976). This model (1) makes fewer assumptions than those using total score, dimension scores or item scores as dependent variables and (2) provides a robust unbiased estimate of a preference-based index from IWQOL-Lite data where a preference-based measure was not used. The addition of age and body mass index (BMI) led to a slight improvement in the model. CONCLUSIONS It is possible to facilitate economic evaluations using results obtained from disease-specific instruments using this approach. A weakness of this approach is that there may be aspects of the condition that have not been properly reflected in the SF-6D index. It is, however, useful when a preference-based measure has not been administered and when it would be impractical to conduct a full valuation survey.