From the Department of Epidemiology, Harvard School of Public Health, and the Harvard-MIT Division of Health Sciences and Technology, Boston, MA.
Editors' note: This series addresses topics that affect epidemiologists across a range of specialties. Commentaries start as invited talks at symposia organized by the Editors. This paper was presented at the 2009 Society for Epidemiologic Research Annual Meeting in Anaheim, CA.
Supported by funds from NIH grant R01 HL080644.
Editors' note: Related articles appear on pages 10 and 3.
Correspondence: Miguel A. Hernán, Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115. E-mail: firstname.lastname@example.org.