Full Text
The Full Text of this article is available as a PDF (990.1 KB).
. 1985;17(3):387–406. doi: 10.1186/1297-9686-17-3-387
Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.