Full Text
The Full Text of this article is available as a PDF (825.8 KB).
. 1990 Mar 15;22(1):93–107. doi: 10.1186/1297-9686-22-1-93
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.