Does parent report measure performance? A study of the construct validity of the Functional Mobility Scale

Aim: Parental report is often relied on to measure performance of activities in children with cerebral palsy (CP). This study examined whether the Functional Mobility Scale (FMS) accurately reflects performance of mobility in children with CP. Method: Eighteen children with spastic CP (11 males, seven females; mean age 12y 8mo, SD 2y 8mo, range 8-17y) were recruited from a special development school. Children were in Gross Motor Function Classification System (GMFCS) levels II (n=5), III (n=4), or IV (n=9), and had quadriplegia (n=9), diplegia (n=7), or hemiplegia (n=2). The children's mobility was observed directly around and outside the home and at school and their mobility methods were recorded. The parent's FMS rating was obtained on the telephone by a physiotherapist. Agreement between direct observation and the FMS rating was examined using quadratic weighted kappa (γ) statistics. Results: Agreement between direct observation and the FMS was as follows: FMS 5m γ=0.71, 45%; FMS 50m γ=0.76, 94%; FMS 500m γ=0.74, 95%. Differences in the range and number of mobility methods were observed by GMFCS level across environmental settings. Interpretation: Substantial agreement was found between FMS ratings and direct observation, particularly over longer distances, providing evidence of the validity of the FMS as a measure of performance in children with CP.