<<I seem to regularly get myself into a muddle with epidemiological =
statistics. In preparation for WBW I need to make sure I get the =
interpretation right. So....can someone help me here. An odds ratio of =
0.46 means that a treatment has halved the likelihood of something =
happening....is that correct?? Just want some confirmation here! Karleen
Gribble Australia>>

Odds ratios are complicated!  An odds ratio is the odds of having an
outcome for those who were exposed to something vs. those who weren't
exposed.  It's used in retrospective studies -- when the outcome has
already happened, and we are looking back in time to learn more about a
risk factor in question.  (An example of a classic retrospective study
that used an odds ratio was one in which young women presented with
vaginal adenocarcinoma, and the researchers noticed that these women had
been prenatally exposed to DES, yielding a high odds ratio for having
the adenocarcinoma for those who had been exposed to DES vs those who
weren't exposed to it.)  On the other hand, prospective studies (you
begin with the risk factors and follow people forward in time, as in the
Framingham Heart Study) yield relative risks.  A relative risk is the
incidence rate of an outcome for a group with and without the risk
factor.  In other words, it's PREDICTIVE: we can directly determine the
excess risk caused by an exposure to a given risk factor.  Odds ratios
are NOT PREDICTIVE.  However, as Lisa Amir said, odds ratios are about
the same as relative risks provided this big assumption: that the event
is rare.  In other words, technically, for frequent outcomes, the odds
ratio is not a good predictor of relative risk.  Hope this explanation
makes sense...

Diane Michel, MSPH, LLLL
Boulder, CO (where genitals = breasts)

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