Application of the Rasch rating scale model to the assessment of quality of life of persons with intellectual disability

Background Most instruments that assess quality of life have been validated by means of the classical test theory (CTT). However, CTT limitations have resulted in the development of alternative models, such as the Rasch rating scale model (RSM). The main goal of this paper is testing and improving the psychometric properties of the INTEGRAL Quality of Life Scale (i.e., fit statistics, person and item reliability coefficients) with RSM. Method The sample comprised 271 Spanish adults with intellectual disability. Results The results showed that the data fitted the model, point-biserial correlations were adequate, items showed precision, reliability of items (.98) and persons (.89) were adequate, and the response categories were suitable (thresholds were ordered). However, one item ("I feel excluded from society") did not fit the model and a few items were too easy for participants to achieve. Conclusions Although it seems necessary to include more difficult items and to delete one, the INTEGRAL Scale is a useful instrument with evidence of validity.