Unidimensionality and reliability under Mokken scaling of the Dutch language version of the SF-36.

The sub-scales of the SF-36 in the Dutch National Study are investigated with respect to unidimensionality and reliability. It is argued that these properties deserve separate treatment. For unidimensionality we use a non-parametric model from item response theory, called the Mokken scaling model, and compute the corresponding scalability coefficients. We estimate reliability under the Mokken model, assuming that the items are double homogeneous, and compare it to Cronbach's alpha. The scalability of the sub-scale general health perceptions is medium (H = 0.46), and for the other sub-scales it is strong (H > or = 0.6). The reliability in terms of alpha indicates that all sub-scales can be used in basic research (alpha > 0.70), but that only physical functioning can be used for clinical applications of quality of life (alpha > 0.90). The relative merits of our approach are discussed.