Development of a short scale for assessing economic environmental aspects in patients with spinal diseases using Rasch analysis

Background: Economic environmental factors represent important barriers to participation and have deleterious effects on quality of life (QOL) in persons with spinal diseases (SpD). While economic factors are anchored in the International Classification of Functioning, Disability and Health, their influence on QOL and participation from patients' perspectives is an infrequent focus of research. The aim of the present research is to calibrate a culturally adapted Rasch-based questionnaire assessing economic QOL in patients with SpD. Methods: The 11-items of the German economic-QOL-scale were answered by 325 patients with SpD on a four-point Likert-scale. Fit to the Rasch measurement model was investigated by testing for stochastic ordering of the items, unidimensionality, local independence, and differential item functioning (DIF). Results: After adjusting for local dependency, fit to the Rasch model was achieved with a non-significant item-trait interaction (chi-square_df=20=34.8, p=0.021). The person separation reliability equaled 0.88, the scale was free from age- or gender-related DIF, and unidimensionality could be verified. Conclusions: The Rasch-based German version of the economic-QOL-scale represents a suitable instrument to investigate the influences of economic factors on patients' QOL at a group and individual level. It can be easily applied in research and practice and may be administered quickly in combination with other instruments. The short test duration implies a low test burden for patients and a minimum of time expenditure by clinicians when evaluating the results.