Inter-rater Reliability and Agreement of the General Movement Assessment and Motor Optimality Score-Revised in a Large Population-Based Sample

Background: Prechtl's General Movement Assessment (GMA) at fidgety age (3-5 months) is a widely used tool for early detection of cerebral palsy. Further to GMA classification, detailed assessment of movement patterns at fidgety age is conducted with the Motor Optimality Score-Revised (MOS-R). Inter-rater reliability and agreement are properties that inform test application and interpretation in clinical and research settings. This study aims to establish the inter-rater reliability and agreement of the GMA classification and MOS-R in a large populationbased sample. Methods: A cross-sectional study of 773 infants from birth-cohort in Perth, Western Australia. GMA was conducted on home-recorded videos collected between 12 + 0 and 16 + 6 weeks post term age. Videos were independently scored by two masked experienced assessors. Inter-rater reliability and agreement were assessed using intraclass correlation coefficient and limits of agreement respectively for continuous variables, and Cohen's Kappa and Gwet's Agreement Coefficient, and percentage agreement respectively for discrete variables. Results: The classification of GMA showed almost perfect reliability (AC1 = 0.999) and agreement (99.9 %). Total MOS-R scores showed good-excellent reliability (ICC 0.857, 95 % CI 0.838-0.876) and clinically acceptable agreement (95 % limits of agreement of +/- 2.5 points). Substantial to almost perfect reliability and agreement were found for all MOS-R domain subscores. While MOS-R domains with higher redundancy in their categorisation have higher reliability and agreement, inter-rater reliability and agreement are substantial to almost perfect at the item level and are consistent across domains. Conclusion: GMA at fidgety age shows clinically acceptable inter-rater reliability and agreement for GMA classification and MOS-R for population-based cohorts assessed by experienced assessors.