Psychometric properties of the RAND-36 among three chronic diseases (multiple sclerosis, rheumatic diseases and COPD) in the Netherlands P. Moorer 1 , Th.P.B.M. Suurmeijer 1,2 , M. Foets 3 & I.W. Molenaar 2,4 1 Northern Centre for Healthcare Research (NCH), University of Groningen (E-mail: p.moorer@med.rug.nl); 2 Interuniversity Centre for Social Science Theory and Methodology (ICS), University of Groningen; 3 NIVEL, Utrecht; 4 Department of Statistics, Measurement Theory and Information, University of Groningen, The Netherlands Accepted in revised form 30 August 2001 Abstract 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 ex- amined. 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 percep- tions’ 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’. Conclu- sions: 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. Key words: Psychometrics, Quality of life, RAND-36 Introduction In the 80’s, the MOS SF-36 [1–3] has been devel- oped as a reasonably short and reliable question- naire to measure different aspects of health, which could be used in clinical practice and research. Besides measuring physical health it was intended to measure mental, social and general health as well. In addition, it should be applicable to dif- ferent populations of impaired or ill persons. Based on factor and reliability analysis [3], the fi- nal multidimensional instrument contained 36 items measuring nine aspects of health: physical functioning, social functioning, role physical, role emotional, mental health, vitality, bodily pain, general health perceptions and health change (see Appendix for items within subscales). Health change has been added at a later stage. Hayset al.[4]presentedanalternativeinstrument to the MOS SF-36, namely the RAND-36, which slightly differs with regard to item summation, but notinthewordingoftheitemsorthestructureofthe instruments.AstheRAND-36canbeconsideredan alternative version of the MOS SF-36, both instru- mentsshouldmeetthesamepsychometricstandards assetbyWareandSherbourne[1].Theyconsidered the most important ones for the MOS SF-36 to be: (1) That the reliabilities should be acceptable. (2) That the inter-item correlation of an item should be higher with its own subscale than with the other subscales in the instrument. In 1991, the international quality of life assessment (IQOLA) project [5–7] in the context of cross-cul- tural translation rules proposed a third standard, namely that the order of item averages should be equal for different countries. Quality of Life Research 10: 637–645, 2001. Ó 2001 Kluwer Academic Publishers. Printed in the Netherlands. 637