For a study, the researchers aimed to establish measurement invariance in the Repetitive Behavior Scales for Early Childhood (RBS-EC), a parent-report questionnaire of RRBs, and model developmental change in RRBs from 8 to 36 months. Restricted and repetitive behaviors (RRBs) are core features of autism spectrum disorder (ASD) and one of the earliest behavioral signs of ASD. Researchers collected RBS-EC responses from parents of typically developing (TD) infants (n=180) from 8 to 36 months (n=606 responses, with participants contributing an average of 3-time points). The RBS-EC showed some non-invariance as a function of age. Researchers were able to adjust this non-invariance to more accurately model changes in the RBS-EC over time. Repetitive motor and self-directed behaviors resulted from a linear decline from 8 to 36 months, while higher-order behaviors showed a quadratic trajectory such that they began to decline later in development at around 18 months. Using adjusted factor scores instead of unadjusted raw mean scores provided several benefits, including increased within-person variability and precision. The RBS-EC was sensitive enough to measure the presence of RRBs in a TD sample, as well as their decline with age. Using factor score estimates of each subscale adjusted for non-invariance allowed researchers to more precisely calculate the change in these behaviors over time.