Background / Context: Children from low-income families benefit remarkably from exposure to compensatory education that began with Head Start in 1965 and aimed to improve school readiness skills by design (Farran, 2007; Scarr & Weinberg, 1986). While empirical evidence has supported more instructional time in elementary and secondary schools for low-income students (Abdulkadiroglu, Angrist, Dynarski, Kane, & Pathak, 2011; Angrist, Dynarsky, Kane, Pathak, & Walters, 2010; Dobbie & Fryer, 2011; Hoxby, Muraka, & Kang, 2009; Patall, Cooper, & Allen, 2010), little is known that whether increasing quantity of Head Start could also benefit low-income children. Also largely unexamined is how Head Start quantity effects differ for different age groups. Research Question: (1) Does the amount of daily exposure to Head Start impact cognitive, pre-academic, and social outcomes? (2) Does the impact vary by age? (author abstract)