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Re: st: Incidence rate ratio


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Incidence rate ratio
Date   Thu, 6 Mar 2008 15:20:01 +0000 (GMT)

--- Mohammed El Faramawi <melfaram@yahoo.com> wrote:
> I am trying to calculate the incidence rate ratio(IRR)
>  for a categorical variable ( Mortality) which  has
> two categories only ( dead or alive) in a probability
> weighted sample. I am also trying to adjust for
> covariates. I know that poisson regression can be used
> to calculate  (IRR) and adjust for covariates given
> that the outcome is  a count for example the number of
>  deaths, number stroke attacks, etc . My question is
> what regression should I use  in such a situation i.e
> outcome which is not count.Can I still use poisson
> regression which I doubt it?? Thank you

--- wangxin wrote:
> so, your dependent variable is dead or alive? that is
> a binary one. you may use Logistic regression.

-logit- will give you odds ratios instead incidence 
rate ratios (or risk ratios as they are often called 
in case of a binary dependent variable). There are 
people who think that odds ratios are too dificult 
to understand, for that reason prefer risk ratios or 
even risk differences. I disagree. For various views 
on this you can follow the thread starting with:
http://www.stata.com/statalist/archive/2008-01/msg00308.html

One problem with risk ratios is that it implies a model 
which can result in  predicted probabilities larger
than 1 if you have a continuous explanatory variable or 
multiple discrete explanatory variables. If you are willing
to live with that, than you can use -poisson- to estimate
this model. Alternatively, you can use -glm- with the 
-family(binomial) link(log)- options. The advantage of the
latter command is that it warns you when your model results
in predicted probabilities larger than 1.

Hope this helps,
Maarten

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------


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