Re: subpopulation in survey library

[Thomas Lumley <tlumley@u.washington.edu>]

If you want the prevelances of current asthma among those with lifetime asthma 
then

svymean(~ASTHNOW, subset(BRFSS.05.svy2, ASTHMA2==1))

should give it to you.

     -thomas

On Tue, 8 Jul 2008, jose Bartolomei wrote:

> Dear S+/R users:
>
>
> I’m still conducting analysis using the survey package, to estimate the prevalence 
> & SE of lifetime asthma (ASTHMA2) and current asthma (ASTHNOW) in complex survey 
> design (stratify design).
>
> I have problems estimating the standard errors when the procedures is 
> implemented in a subpopulation of the entire universe.
>
> The procedure goes as follows:
>
>
> BRFSS.05.svy<-svydesign(ids=~0, strata=~STSTR, weights=~FINALWT, 
> data=BRFSS.05.F)
>
> BRFSS.05.svy<-as.svydesign2(BRFSS.05.svy)
>
> svymean(~ASTHMA2,design=BRFSS.05.svy)
>              mean     SE
> ASTHMA21 0.1928144 0.0082
> ASTHMA22 0.8061478 0.0082
> ASTHMA27 0.0010377 0.0005
>
> ; which gives the correct prevalence and standard error estimators.
>
> Then want to calculate the current prevalence, which is a subpopulation of 
> ASTHMA2.
>
> I use the following procedure
>
>
> Nw.05<-sum(BRFSS.05.F$FINALWT)# the weighted population
>
> AnowPrev<-(svytotal(~ASTHNOW, design=BRFSS.05.svy2, na.rm=TRUE)/Nw.05)*100
>
> > test
>             total      SE
> ASTHNOW1  8.792641 15761.4
> ASTHNOW2 10.456593 19130.4
> ASTHNOW7  0.032211   664.2
>
> Or
>
> subtest.optB<-subset(BRFSS.05.svy2, ASTHNOW==1)
> (svytotal(~ASTHNOW, subtest)/Nw.05)*100
>
> Total      SE
> ASTHNOW1  8.792641 15761.4
> ASTHNOW2 10.456593 19130.4
> ASTHNOW7  0.032211   664.2
>
> ; which gives the correct prevalence but incorrect standard error estimators.
>
> How can I get the correct SE’s?
>
> Thanks in advance
>
>
> Jose
> PS: Bellow are 2 examples of various procedures that I tried that give 
> incorrect results.
> > svymean(~ASTHNOW, design=BRFSS.05.svy2)
>         mean SE
> ASTHNOW1   NA NA
> ASTHNOW2   NA NA
> ASTHNOW7   NA NA
>
>
> > svymean(~ASTHNOW, design=BRFSS.05.svy2, na.rm=TRUE)
>              mean     SE
> ASTHNOW1 0.4560157 0.0231
> ASTHNOW2 0.5423138 0.0231
> ASTHNOW7 0.0016706 0.0012
>
>
> _________________________________________________________________
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Thomas Lumley                   Assoc. Professor, Biostatistics
tlumley@u.washington.edu        University of Washington, Seattle