Psychometric properties of the RAND-36 among three chronic diseases (multiple sclerosis, rheumatic diseases and COPD) in The Netherlands.

OBJECTIVE: In this article, psychometric properties both of the total RAND-36 and of its subscales, such as unidimensionality, differential item functioning (DIF or item bias), homogeneity and reliabilities, are examined. METHODS: The data from populations with three chronic illnesses, multiple sclerosis (n = 448), rheumatism (n = 336) and COPD (n = 259), have been collected in different parts of the Netherlands. The main technique used was Mokken scale analysis for polytomous items. RESULTS: All subscales of the RAND-36 appeared to be unidimensional. For the subscales 'mental health' and general health perceptions' some minor indications of DIF for the different chronic illnesses were found. Reliabilities of almost all subscales in all subpopulations were higher than 0.80, while the homogeneities of almost all subscales in all subpopulations were higher than 0.50, indicating 'strong unidimensional, hierarchical scales'. CONCLUSIONS: In general, the subscales of the RAND-36 can be used to compare persons with different chronic illnesses. The subscale 'general health perceptions' did not function as well as would be preferred.

An application of Mokken's model for stochastic, cumulative scaling in psychiatric research.

Rating scales used in psychiatric research are often based on factor analysis. In practice, the resulting factors or dimensions will generally be empirically derived rather than based on theoretical considerations. In addition, it often proves difficult to determine their validity. It is argued that the use of an hierarchical scale may be a partial answer to these problems. This is illustrated by the analysis of data from two interview schedules, using Mokken's model for stochastic, cumulative scaling. Data were derived from a follow-up study of patients with a first life-time episode of non-affective, functional psychosis. Two scales for the assessment of psychopathology and social dysfunctioning were constructed. Scale statistics proved to be satisfactory, while the order of items in the scales corresponded to a priori theoretical expectations.