Brownson, Ross C., and Christine M. Hoehner. "Epidemiology: A Foundation of Public Health." Applied Epidemiology: Theory to practice. : Oxford University Press, 2006-08-17. Oxford Scholarship Online. 2009-09-01. Date Accessed 22 May. 2013 <http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195187410.001.0001/acprof-9780195187410-chapter-1>.
Brownson, R., & Hoehner, C.(2006-08-17). Epidemiology: A Foundation of Public Health. In Applied Epidemiology: Theory to practice. : Oxford University Press. Retrieved 22 May. 2013, from http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195187410.001.0001/acprof-9780195187410-chapter-1.
Brownson, Ross C., and Christine M. Hoehner. "Epidemiology: A Foundation of Public Health." In Applied Epidemiology: Theory to practice. : Oxford University Press, 2006-08-17. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195187410.001.0001/acprof-9780195187410-chapter-1.
Brownson RC, Hoehner CM. Epidemiology: A Foundation of Public Health. In: Brownson RC, Petitti DB. Applied Epidemiology: Theory to practice. Oxford University Press; 2006. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195187410.001.0001/acprof-9780195187410-chapter-1. Accessed May 22, 2013.