Longitudinal Measurement Invariance of the Autism Impact Measure (AIM) in Children and Adolescents With Autism.

PURPOSE: The Autism Impact Measure (AIM) is a caregiver-reported measure that assesses behavioral changes in response to treatment in children with autism. This study evaluated its longitudinal measurement invariance, which is critical for ensuring that observed score changes reflect true behavioral change rather than construct-irrelevant variance. METHODS: The sample comprised a large clinical cohort from the Autism Treatment Network (ATN) Registry (N = 597; baseline Mage = 9.7 years, SD = 3.3). Using multigroup confirmatory factor analysis and established fit criteria, we examined the longitudinal measurement invariance of the AIM scores among participants with longitudinal data (n = 371) approximately 1 year later (M = 1.17 years; SD = 0.28). RESULTS: The AIM scores demonstrated longitudinal configural, metric, and scalar measurement invariance within participants across the one-year interval for all symptom domains: Repetitive Behavior, Communication, Atypical Behavior, Social Reciprocity, and Peer Interaction. CONCLUSION: These findings provide validity evidence for the interpretation that observed changes in AIM scores across time reflect genuine differences in autism symptom presentation rather than shifts in item functioning, thereby further supporting the AIM's use as a longitudinal symptom tracking tool in autism treatment.