Functional Symmetry Observation Scale, Version 2: discriminant validity and recommendations for the development of
Mary Rahlin1, Steven Miller2, Bernadette Sarmiento3
1Board-Certified Clinical Specialist in Pediatric Physical Therapy, Department of Physical Therapy, Rosalind Franklin University of Medicine and Science, North Chicago, IL, USA.
Physiotherapy Theory and Practice
|December 3, 2025
Summary
The Functional Symmetry Observation Scale-V2 (FSOS-V2) shows scoring bias affecting infant symmetry assessment. Revisions are recommended to improve its reliability and validity for clinical use in congenital muscular torticollis.
Area of Science:
- Pediatric Physical Therapy
- Infant Motor Development
- Clinical Assessment Tools
Background:
- The Functional Symmetry Observation Scale-V2 (FSOS-V2) is a video-based tool for assessing infant movement symmetry in congenital muscular torticollis.
- While possessing good validity and intrarater reliability, the FSOS-V2 exhibits poor to moderate interrater reliability.
- Previous data suggested potential scoring bias related to infant age and motor development, impacting the scale's discriminant validity.
Purpose of the Study:
- To evaluate the discriminant validity of the FSOS-V2.
- To conduct individual item analyses using existing reliability data.
- To develop recommendations for modifying the FSOS-V2 into a shorter, more reliable form.
Main Methods:
- Secondary analysis of FSOS-V2 reliability data from 50 infants with congenital muscular torticollis (CMT).
- Correlation analyses between FSOS-V2 scores, infant age, and gross motor milestones.
- Item response theory (IRT) modeling for individual item analysis and scale modification recommendations.
Main Results:
- One rater's scores significantly correlated with infant age and motor milestones, indicating potential scoring bias.
- IRT modeling provided specific recommendations for item removal and scale modification.
- Evidence of scoring bias impacting discriminant validity and interrater reliability was confirmed.
Conclusions:
- Scoring bias in the FSOS-V2 compromises its discriminant validity and interrater reliability.
- Revising the scale into a shorter form and refining scoring guidelines are necessary.
- These modifications are crucial for accurate and consistent assessment of symmetry in infants with CMT.
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