Construct Validity of the Physical Function Scale of the Swiss Spinal Stenosis Questionnaire for the Measurement of Walking Capacity

Study Design. Measurement (validity) study using data from a prospective longitudinal study of lumbar spinal stenosis. Objective. Provide convergent and divergent validity evidence for the use of the Physical Function Scale of the Swiss Spinal Stenosis Questionnaire for the measurement of walking capacity in persons with lumbar spinal stenosis. We were also interested in the association between the Physical Function Scale and the Oswestry Disability Index (ODI). Summary of Background Data. The Physical Function Scale has been used to assess walking capacity in persons with lumbar spinal stenosis; however, there have been limited studies investigating its psychometric properties. No validity studies have compared the Physical Function Scale and the ODI head to head. Methods. The Physical Function Scale was correlated with the ODI, Health Utilities Index, Centres for Epidemiologic Studies Depression Scale, Medical Outcomes Survey Social Support Scale, and an additional item from the Oxford Claudication Score. Results. As hypothesized, the Physical Function Scale was correlated highly with those instruments and items intended to measure walking capacity and minimally with those instruments intended to measure different constructs. The correlation between the Physical Function Scale and the ODI was r = 0.719. Conclusion. Results support construct validity of the Physical Function Scale for the measurement of walking in an lumbar spinal stenosis population. However, it cannot be ascertained from the current study that the construct being measured is, indeed, walking capacity. Further research is warranted to investigate criterion validity evidence for the use of the Physical Function Scale in the measurement of walking capacity in lumbar spinal stenosis, by examining the relationships between self-report and observational measures of walking.