Measuring Quality of Life After Stroke Using the SF-36

The SF-36 is the most widely used generic instrument for measuring quality of life (QOL). The instrument is translated into numerous languages, and the validity of the 8 subscales is confirmed in general populations and in a wide variety of patient groups in more than 2000 articles. In an article published in this issue of Stroke, Hobart et al1 report the psychometric properties of the SF-36 in a sample of ischemic stroke patients. The authors conclude that (1) some subscales, especially the scales for General Health (GH) and Social Functioning (SF), have limited reliability and validity; (2) half of the subscales suffer from floor and/or ceiling effects; and (3) the 2 summary scores inadequately reflect the patient’s physical and mental health. In view of the overwhelming weight of evidence that the subscales of the SF-36 are psychometrically sound to measure QOL in a range of patient populations, the question arises how convincing the arguments of Hobart and his colleagues are. The authors argue that the GH and SF scales generate low reliability scores and have limited convergent and discriminant validity. However, these conclusions can be challenged. The reliability of only 1 scale (GH) was marginally less (Cronbach’s alpha 0.68) than the authors’ predefined criteria. Although it is often recommended that coefficient values should be above 0.80, values above 0.70 are generally regarded as acceptable for scales when assessing outcome on a group level. Moreover, it should be noticed that the alpha coefficient not only depends on the correlations of the items but is also related to the number of items in the scale. For example, the relatively low coefficient (0.70) of the SF scale may also be explained by the fact that this scale encompasses only 2 items. Since Cronbach’s coefficients increase as the number of items is increased (or vice versa), one may wonder whether it is sensible to specify criteria for acceptable levels of the alpha coefficient without specifying the number of items in the scale.2 The authors’ criticism with regard to the convergent and discriminant validity of both subscales is not convincing. In general, the item-total subscale correlation of the GH and SF is above 0.40, indicating that the items in each scale measure a common underlying trait. Moreover, in both scales, all item-own scale correlations are higher than the item-other scale correlations (although less than 2 SE), See article on page 1349