An official website of the United States government
The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before
sharing sensitive information, make sure you’re on a federal
government site.
The site is secure.
The https:// ensures that you are connecting to the
official website and that any information you provide is encrypted
and transmitted securely.
Whitehead A, Whitehead J.Whitehead A, et al. Among authors: whitehead j.Stat Med. 1991 Nov;10(11):1665-77. doi: 10.1002/sim.4780101105.Stat Med. 1991.PMID: 1792461