Department of Epidemiology and Public Hearth / Nuffield Department of Clinical Medicine / Epidemiological Studies Unit / Johns HopWns University School of Hygiene and Public Health ADrightsreserved Vo / University College London Medical School / office of National Statistics / Clinical Trial Service Unit / London School of Hygiene / /
American Journal of Epidemiology Copyright O 1999 by The Johns HopWns University School of Hygiene and Public Health ADrightsreserved Vo). 150, No. 4 Printed hi U.S.A