Two weeks ago, researchers at Duke University released preliminary results of a study about pre-kindergarten programs in North Carolina. According to the press release, "North Carolina third-graders have higher standardized reading and math scores and lower special education placement rates in those counties that had received more funding for Smart Start and More at Four when those children were younger."
A full report is not available, so we do not have access to many of the specifics of the study. But researchers have released a Power Point presentation with a summary of the sample and methodology. For example, the study sample includes "all children of the appropriate age in a county when the program is available, regardless of whether a child participated directly." One of the limitations of the study is that it does not include "all children who have access to the program" because the researchers "lose some of them and treatment status for some is unclear."