The field knows little about the specific program design factors that lead to favorable program outcomes (Reynolds, 2004), and very few studies have addressed this issue. Therefore, we reviewed two frequently cited studies that looked at Head Start programs, the nation's largest early educational intervention, and examined the impact of one program factor, intervention dosage, on children's school readiness outcomes. In the two studies, program dosage was defined as the amount of services children received, and was measured as the duration of program enrollment (i.e., one year versus two years) and the intensity of the program (i.e., half day versus full day). (author abstract)