a 2010 report by the department of health and human services, "head start impact", examined the cognitive development, social-emotional development, and physical health outcomes of 4,667 three- and four-year-old children in a nationally representative sample of programs across 23 states. children were randomly assigned to either a head start group (participants) or a non-head start group (control group). the children in the two groups were similar in all measured characteristics at program entry. pre-participation assessments of all critical outcome measures were taken. control group children optionally enrolled in non-head start programs. nearly half of the control-group children enrolled in other preschool programs. outcome measures covered cognitive development, social-emotional development, health status and access to health care, and parenting practices. head start students were split into two cohorts – 3-year-olds with two years of head start and 4-year-olds with one year of head start. the study found: