From the Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD.
NOTES
Reprint requests to Dr. Alfred Sommer, Johns Hopkins Bloomberg School of Public Health, 615 North Wolfe Street, Suite W1041, Baltimore, MD 21205 (e-mail address: asommer{at}jhsph.edu).
Received for publication July 17, 2000. Accepted for publication November 3, 2000.