# Re: st: Help with confidence intervals generated with nlcom fromsurvey data

 From Arnold Kester <[email protected]> To [email protected] Subject Re: st: Help with confidence intervals generated with nlcom fromsurvey data Date Fri, 08 Jul 2005 17:08:32 +0200

I can imagine that NLCOM stubles on this. A more "by hand" procedure would be based on the following rules, of which c) is approximate. You need the standard errors of the estimated rates.

a) log(rate1/rate2)= log(rate1)-log(rate2).

b) s.e.(log(rate1)-log(rate2))
= sqrt( s.e.(log(rate1)^2+s.e.(log(rate2)^2 )

c) s.e.(log(rate1)) = (1/rate1) * s.e.(rate1)

Apply rule c to get a standard error for log(rate), seperately for rate1 and rate2. Then rule b to get the s.e. of log(rate1/rate2). Then compute a confidence interval for log(rate1/rate2):

log(rate1/rate2) +- 1.96 * se

transform lower and upper bounds using exp.

Hope this helps,
Arnold Kester

Op 07/07/2005 06:50 PM schreef Bird, Tommy M "Mac":

```Hello,
I am calculating rates of a particular disease in two different groups
using survey data.  I am using SVYRATIO for this and it is working
nicely.  Now I would like to create a rate ratio by dividing the rate
from one group by the rate of the other group.  I can do this with NLCOM
and it works nicely.  My problem is that the confidence intervals for my
rate ratio are not logit transformed, thus some of my lower confidence
intervals are negative.  I really need a confidence interval that cannot
go below zero.  How can I do this?  Is their a transformation that I can
run on the ends of the confidence intervals?  Is their an option that I
am not aware of on NLCOM?  Is there a better way to do this all
together?  Please keep in mind that I am neither a statistician, or a
Stata expert.  Thanks so much,

Mac Bird, MS
Research Associate
Child Health Services Research Group
Center for Applied Research and Evaluation
Department of Pediatrics
University of Arkansas for Medical Sciences

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```--
Met vriendelijke groet,
Arnold Kester

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