DEMQOL and DEMQOL-Proxy: a Rasch analysis.

BACKGROUND: DEMQOL and DEMQOL-Proxy are widely used patient reported outcome measures (PROMs) of health related quality of life in people with dementia (PWD). Growing interest in routine use of PROMs in health care calls for more robust instruments that are potentially fit for reliable and valid comparisons at the micro-level (patients) and meso-level (clinics, hospitals, care homes). METHODS: We used modern psychometric methods (based on the Rasch model) to re-evaluate DEMQOL (1428 PWDs) and DEMQOL-Proxy (1022 carers) to ensure they are fit for purpose. We evaluated scale to sample targeting, ordering of item thresholds, item fit to the model, and differential item functioning (sex, age, relationship), local independence, unidimensionality and reliability on the full set of items and a smaller item set. RESULTS: For both DEMQOL and DEMQOL-Proxy the smaller item set performed better than the original item set. We developed revised scores using the items from the smaller set. CONCLUSIONS: We have improved the scoring of DEMQOL and DEMQOL-Proxy using the Rasch measurement model. Future work should focus on the problems identified with content and response options.