As I discuss in my just-published book, From Preschool to Prosperity, critics often argue that the research evidence for preschool only comes from small-scale programs run by researchers. Therefore, the argument goes, we don’t know if preschool can work if run at a large-scale by real-world government agencies.

But we actually do have good evidence that preschool can work at a large scale. As mentioned in a previous post, we have good evidence for the effectiveness of preschool in programs run at a large scale in Chicago, Boston, and Tulsa, and in many states. This evidence is not random assignment evidence – it would be quite difficult and expensive to run a random assignment experiment in a large-scale ongoing program run by a public agency. But this evidence comes from studies that have good comparison groups, with access to the program varying due to “natural experiments”, which argues that the estimated effects on adult outcomes and test scores represent true effects of the program, not pre-existing differences between program participants and non-participants.

The programs with good evidence are also programs run by a variety of public agencies. For example, in Chicago, Boston, and Tulsa, these preschool programs are run by the public schools in these cities. These preschools operate in diverse political, cultural, and economic situations, which suggests that preschool does not require special circumstances to be successful.

Preschool works. And we know this not just from small-scale “hothouse” experiments, but from large-scale implementation of preschool in many places across the U.S.