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.
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.
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'.
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.