Objective: The present study aimed to determine inter-rater reliability and intra-rater reliability of the 66-item version of the Gross Motor Function Measure (GMFM-66) in assessing motor ability of Thai children with cerebral palsy.
Material and Method: Ten children, aged 2-10 years, were recruited in the present study. Eight children with spastic diplegia from an outpatient department of Thammasat University Hospital, and two normal children were recruited. The motor abilities of the subjects were recorded on a video camera. Three pediatric physical therapists independently watched the video recordings and scored each child according to the GMFM assessment. The video clips of ten children were assessed on two occasions at an interval of one week by the same therapists. The inter-rater and intra-rater reliability were analyzed using an intraclass correlation coefficient (ICC).
Results: The inter-rater reliability and intra-rater reliability of the GMFM-66 were both high. The ICC for inter-rater reliability was 0.93 and for intra-rater reliability were 0.99-1.00 for the total scores.
Conclusion: The present study confirms high inter-rater and intra-rater reliability of the GMFM-66 measured by Thai pediatric physical therapists. Therefore, in clinical practice, we recommend Thai pediatric physical therapists to use the GMFM-66 in assessing motor ability of cerebral palsy patients.

Keywords: Cerebral Palsy, Gross Motor Function Measure, Inter-rater reliability, Intra-rater reliability