Read Executive Director Liane Akana's research review on the summer slump, which references several of these sources.
Find brief cited statistics at www.summerlearning.org.
The Learning Season: Executive Summary by Dr. Beth M. Miller.
Cooper, Harris et. al. (1996) The Effects of Summer Vacation on Achievement Test Scores: A Narrative and Meta-Analytic Review. Review of Educational Research, 66(3) 227-268. See abstract
Alexander, Karl L., Entwisle, Doris R. & Olson, Linda Steffel. (2007) Lasting Consequences of the Summer Learning Gap. American Sociological Review, 72(2) 167-180. See abstract
Education Commission of the United States. (June 2009) Summer Learning; Moving from the Periphery to the Core. The Progress of Education Reform, 10(3) 1-6.