Julie,
Your description of the data and question are ambiguous. The simplest
interpretation says that if you contracted HIV from 1989-1995 the odds are
53/46 = 1.15 that you contracted it from a low risk partner. From 1995-2000
the odds were 90/19 = 4.74. Thus, the proportion of people with HIV who
contracted it from a low risk partner was 4.74/1.15 = 4.12 greater in the
latter period than in the former. This means there must be many more low
risk carriers in the latter time period.
However, these higher numbers contracting HIV from low risk partners does
not mean that a low risk partner is more likely to infect than a high risk
partner. It is simply that there are apparently many more low risk HIV
carriers in the population than high risk carriers, so naturally they infect
more people. But faced with a choice between a high risk partner and a low
risk partner, the relative odds of being infected by each cannot be
calculated from the data you have given. One would have to know how many
people had high or low risk partners and did not become infected. If you
construct 2 x 2 tables for each time period, you will see that you are
missing all of the information on the partners of people who did not
contract HIV.
David L. Doggett, Ph.D.
Senior Medical Research Analyst
Health Technology Assessment and Information Services
ECRI, a non-profit health services research organization
5200 Butler Pike
Plymouth Meeting, Pennsylvania 19462, U.S.A.
Phone: (610) 825-6000 x5509
FAX: (610) 834-1275
http://www.ecri.org
e-mail: [log in to unmask]
-----Original Message-----
From: Julie Clark [mailto:[log in to unmask]]
Sent: Thursday, August 02, 2001 6:11 AM
To: [log in to unmask]
Subject: odds ratios
Dear colleagues,
Apologies for my previous e-mail - I
misunderstood the question! What I need to know is how much more likely you
would be to acquire HIV infection with a non- high risk partner in 1995-2000
as opposed to 1990-1995. The figures are as follows:
high risk partner nonhigh risk partner
1989-1995 46 53
1995-2000 19 90
Many thanks,
Julie Clark
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