https://www.fcd-us.org/promoting-school-readiness-in-oklahoma-an-assessment-of-tulsas-prekindergarten-program/
Since the mid-1990s, three states, including Oklahoma, have established a universal PreKindergarten (PreK) program. We analyze the effects of Oklahoma’s universal PreKindergarten (PreK) program for four-year-olds on children in Tulsa Public Schools (TPS).
The main difficulty with testing the causal impact of a voluntary PreK program is that certain parents are more likely to select PreK, and these parents might have other unobservable characteristics that influence the test outcomes of their children. Because TPS administered an identical test in September 2001 to children just beginning PreK and children just beginning kindergarten, we can compare test outcomes of old PreKindergarten students to test outcomes of young kindergarten students who attended PreK the previous year. We find that the Tulsa PreK program increases cognitive/knowledge scores by approximately 0.39 standard deviation, motor skills scores by approximately 0.24 standard deviation, and language scores by approximately 0.38 standard deviation. Impacts tend to be largest for Hispanics, followed by blacks, with little impact for whites. Children who qualify for a free lunch have larger impacts than other children.