To establish the internal consistency and validity of an observational method for assessing disability in mobility in patients with osteoarthritis (OA).
Data were obtained from 198 patients with OA of the hip or knee. Results of the observational method were compared with results of self-report methods (questionnaires) on disability in mobility.
Both Cronbach's alpha and Mokken Scale Analysis indicate that the method is internally consistent. Using factor analysis, observed and self-reported disability in mobility were found to be closely associated and could not be differentiated.
The observational method is internally consistent and indeed measures disability in mobility (high convergent validity), but observations and self-report seem to yield largely equivalent information (low divergent validity). This raises questions regarding the simultaneous use of both observational and self-report methods in the assessment of disability in mobility in OA patients.