Revision of the ICIDH Severity of Disabilities Scale by data linking and item response theory

The Severity of Disabilities Scale (SDS) of the ICIDH reflects the degree t o which an individual's ability to perform a certain activity is restricted . This paper describes the application of two models from item response the ory (IRT), the graded response model and the partial credit model, in order to derive a tentative proposal for a revised SDS. The key ingredient of th e approach is to scale existing disability items obtained in different stud ies on a common scale by exploiting the overlap. Both IRT models are fitted to a linked data set containing items for measuring walking disability. Ba sed on these solutions, a tentative SDS is constructed. The paper concludes with a discussion of the implications, limitations and advantages of the a pproach. Copyright (C) 2001 John Wiley & Sons, Ltd.