I was recently asked how pre-K’s effects compared with the usual income gaps in kindergarten readiness. How much can high-quality pre-K do to help children from low-income families catch up to children from more middle-class families?
My paper with Gormley and Adelstein on Tulsa pre-K provides an answer to this question. In this paper, the average test score gap at kindergarten entrance between children eligible for a free lunch, and children who must pay full price, is 14 percentile points.
Half-day pre-K for one school year raises test scores of low-income children about 12 percentile points. Full-day pre-K for one year raises test scores of low-income children about 18 percentile points.
Therefore, research suggests that even one-half day of pre-K for only one school year will close most of the gap across income groups in kindergarten readiness. A full-day of pre-K more than closes these income gaps.
Of course, if pre-K is universal, these income gap-closing features are reduced. Universal pre-K helps children from all income groups, but has stronger percentage effects for children from low-income families. But universal pre-K will help raise the overall achievement levels, productivity, and economic future of the entire economy, which is a goal worth pursuing. Universal pre-K increases the size of the overall economic pie, while helping low-income groups the most in percentage terms. Universal pre-K advances both economic efficiency and equity goals.
investinginkids 
Interesting post Tim! Here in DC, we’re in the process of implementing full-day universal pre-K and there is already some interesting data coming out about the edge students who attended pre-K have in 3rd grade tests – in a few years it would be amazing to see expanded public pre-K close the income gap as dramatically as you suggest here. http://dcactionforchildren.org/content/dc-students-who-participated-pre-k-have-edge-third-grade
Thanks for your comments, Bonnie. And thanks for the links to your information on D.C. Do you have more details on how the comparison group was constructed in this D.C. study?
In this case I think it was a pretty simple reporting of results on our statewide 3rd grade tests in 2012 that differentiated between students who had attended public pre-k and those who had not – you can see more on page 13 of this presentation. Our state agency reported differences in pre-k vs. non pre-k proficiency among racial groups and economically disadvantaged students. Unfortunately their analysis doesn’t go so far as to control for these factors – and others – to give a better picture of pre-k’s effects. Hopefully once we have a longitudinal data system with publicly accessible data, some more rigorous studies can be done. http://osse.dc.gov/sites/default/files/dc/sites/osse/release_content/attachments/DCCAS_2012.pdf