On "Prospective Evaluation of the AM-PAC-CAT..." Jette et al. Phys Ther. 2007;87:385-398.

To the editor: The authors are to be complimented for a strong paper describing a methodologically complex process in an understandable manner. Although others have applied computerized adaptive testing (CAT) applications in outpatient rehabilitation for several years, 1 Jette et al 2 are the fi rst to publish results of a practical application of a CAT in a peer-reviewed journal. The strength of the work by Jette et al lies in the process used to develop the product. Item response theory (IRT) methods and CAT applications have the potential to be the foundation of outcomes measurement development in rehabilitation just as they were in educational measurement. 3 We should not forget that IRT and CAT are not new; they are just new to rehabilitation and medicine. Jette et al discuss in the current paper and in earlier work how these methods can be used to develop a new outcomes scale, assess the strengths and weaknesses of the scale, and discuss how a scale can be improved when scale defi cits are identifi ed via practical application in busy clinics. These methods are sorely needed for many common paper-and-pencil instruments that are so popular in rehabilitation. The study is not without limitations , many of which are detailed nicely by Jette et al. One psycho-metric issue not discussed relates to differential item functioning (DIF). 4 Differential item functioning occurs when patients from different groups—for example, patients with hip versus knee im-pairments—have different probabilities of endorsing item response categories. In clinical terms, that means patients with knee impairments perceive the act of squatting as more diffi cult compared with patients who have hip impairments , which is clinically logical and important. 5 Differential item functioning is common in patients treated in outpatient rehabilitation. 5,6 When DIF is present and of practical importance, the lack of control for DIF can erode the validity of the outcomes measure. 3 One of the strengths of IRT techniques is the ability to detect and possibly control for DIF. However , when DIF is identifi ed but of no practical importance, DIF can be ignored when calibrating items. 7,8 Discussion of DIF at least by body part treated would have strengthened the Jette et al paper, particularly because differences in item calibrations by body part treated 6 have been published for the physical functioning items of the SF-36, 9 which appear to be included in the AM-PAC-CAT …