Using Rasch analysis to form plausible health states amenable to valuation: the development of CORE-6D from a measure of common mental health problems (CORE-OM)

Purpose To describe a new approach for deriving a preference-based index from a condition specific measure that uses Rasch analysis to develop health states. Methods The CORE-OM is a 34-item instrument monitoring clinical outcomes of people with common mental health problems. The CORE-OM is characterised by high correlation across its domains. Rasch analysis was used to reduce the number of items and response levels in order to produce a unidimensional measure and to generate a credible set of health states corresponding to different levels of symptom severity using the Rasch item threshold map. Results The proposed methodology resulted in the development of CORE-6D, a 2-dimensional health state descriptive system consisting of a unidimensional 5-item emotional component (derived from Rasch analysis) and a physical symptom item. Inspection of the Rasch item threshold map of the emotional component helped identify a set of 11 plausible health states, which, combined with 3 physical symptom item levels, form 33 plausible health states that can be used for the valuation of the instrument, resulting in the development of a preference-based index. Conclusions This is a useful new approach to develop preference-based measures from existing instruments with high correlations across domains. The CORE-6D preference-based index will enable calculation of Quality-Adjusted Life Years in people with common mental health problems.