Functional Symmetry Observation Scale, Version 2: discriminant validity and recommendations for the development of short forms.

BACKGROUND

The Functional Symmetry Observation Scale (FSOS-V2) is a video-based instrument that measures symmetry in spontaneous movement and posture in infants with congenital muscular torticollis. It has established content and construct validity and acceptable intrarater reliability. However, its interrater reliability is poor to moderate, which precludes the scale use by several therapists assessing symmetry in the same child. While the FSOS-V2 scores should be independent of the infants' age and motor development, visual inspection of reliability data obtained in previous research indicated a potential scoring bias. Younger infants received lower scores from at least one rater, with potential implications for discriminant validity of the scale. Additionally, excessive scoring time and discrepancies in score interpretation among raters likely contributed to lower-than-expected interrater reliability, leading to a recommendation to develop a shorter form of the FSOS-V2.

PURPOSE: The purposes of the current study were to evaluate the discriminant validity of the FSOS-V2, perform individual item analyses using the existing reliability data, and develop recommendations for scale modifications into a short form.

METHODS

Secondary analyses of FSOS-V2 reliability data obtained by four raters from 50 infants with CMT were performed. Participants' videos were examined for the number of achieved major gross motor milestones. Correlations between the FSOS-V2 scores and participants' age, between the FSOS-V2 scores and the number of achieved milestones, and between the age and the number of achieved milestones were calculated. Individual item analysis was performed using descriptive statistics and item response theory (IRT) modeling.

RESULTS

One rater's scores correlated significantly with participants' age and number of achieved milestones. IRT modeling yielded recommendations for scale modifications and removal of several items.

CONCLUSION

Scoring bias affected the discriminant validity and interrater reliability of the FSOS-V2. Current results corroborate scale revisions into short forms and modifications to scoring guidelines.