Advancing the construct validity of the Early Communication Indicator (ECI) for infants and toddlers: Equivalence of growth trajectories across two early head start samples

•Examined invariance in the four key skills and total communication growth trajectories of the Early Communication Indicator (ECI).•Invariance is an important step in development because a comparison between samples assumes that the measurements are factorially invariant.•Vocalizations, single and multiple words, and total communication trajectories all met strong factorial and structural invariance.•Gestures met only weak structural invariance likely due to updates in coding definitions and a website upgrade affecting one sample.