Snyder, Thomas D.; Hoffman, Charlene M. (2001). Digest of Education Statistics 2001(PDF). Institute of Education Sciences, Washington: National Center for Education Statistics. p. 7, fig. 1. Archived(PDF) from the original on 9 August 2017. Retrieved 19 August 2017.
Snyder, Thomas D.; Hoffman, Charlene M. (2001). Digest of Education Statistics 2001(PDF). Institute of Education Sciences, Washington: National Center for Education Statistics. p. 7, fig. 1. Archived(PDF) from the original on 9 August 2017. Retrieved 19 August 2017.