The severity of autism spectrum disorder (ASD) is often measured by co-occurring conditions, such as intellectual disability or language delay, rather than deficits in social interaction, and restricted interests and repetitive behaviors. The Autism Diagnostic Observation Schedule calibrated severity score (ADOS CSS) was created to facilitate comparison of the diagnostic features of ASD independent of related conditions over time. We examined the relationship between the ADOS CSS, ADOS total score, and clinician rated degree of impairment (DOI) in the Study to Explore Early Development. Like others, we confirmed that, among the measures we evaluated, the ADOS CSS was least influenced by developmental functioning and demographic factors and is therefore the best measure of core features of ASD in pre-school children.