   
Hi R-users!

I have a problem with the survey package and i would be very grateful if you can help me.

A short example:  

stratum id weight nh Nh  y sex
     1      1      3     5 15 23   1
     1      2      3     5 15 25   1
     1      3      3     5 15 27   2
     1      4      3     5 15 21   2
     1      5      3     5 15 22   1
     2      6      4     3 12 33   1
     2      7      4     3 12 27   1
     2      8      4     3 12 29   2

 where nh is size of sample stratum and Nh the corresponding  population value, and  y is  metric variable.

Now if i let

 design <- svydesign( id=~1, data=age, strata=~stratum, fpc=~Nh)
then weights(design)  gives me 3,3,3,3,3,4,4,4.

If i then let

x<- postStratify( design, strata=~sex, data.frame(sex=c("1","2"), freq=c(10,15)))
 the weights become 

 1                    2            3            4            5            6            7            8 
2.17               2.17       5.35       5.35        2.17    1.73        1.73        4.28

If i define

design <- svydesign( id=~1, data=age )
x<- postStratify( design, strata=~sex, data.frame(sex=c("1","2"), freq=c(10,15)))
weights become  2 2 5 5 2 2 2 5

The question: does poststratify recognize that i have already stratified in the first design by stratum and then it post stratifies by sex? and why is that? (because i don't have the full joint distribution, the sex*stratum crossing, in order to apply correctly the post stratify function)
I see that Mr Lumley uses the postStratify function when the design does not include strata (eg from ?poststratify: 

dclus1<-svydesign(id=~dnum, weights=~pw, data=apiclus1, fpc=~fpc) rclus1<-as.svrepdesign(dclus1) 
rclus1p<-postStratify(rclus1, ~stype, pop.types) 

and  i use
design <- svydesign( id=~1, data=age, strata=~stratum, fpc=~Nh)
x<- postStratify( design, strata=~sex, data.frame(sex=c("1","2"), freq=c(10,15)))
 
which has a first strata (stratum from svydesign) and a second strata(sex, from poststratify)
Is it correct to use the functions as use them or am i doing something wrong?


Thank you !

       
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