Using Rasch analysis to form plausible health states amenable to valuation: the development of CORE‐6D from CORE‐OM in order to elicit preferences for common mental health problems

I Mavranezouli, J E Brazier,T A Young, M Barkham

ABSTRACT:

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 reference-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.