A study of the impact of ExCELL (Exceptional Coaching for Early Language and Literacy), a professional development intervention for Head Start teachers, on the vocabulary gap between children with higher and lower initial vocabulary skill levels, and of the differential associations of aspects of the intervention experience, in particular structural and process quality, to children's vocabulary development depending on their initial vocabulary skills, based on observations of 19 intervention and 11 control classrooms and on assessments of 358 intervention and 183 control children, all from either 2 randomly assigned intervention Head Start grantees or from 1 control grantee