# Post stratification weights and the survey package in R

I have survey data that needs to be weighted, and to help me with this task, I have access to the full joint distributions of the variables I want to use. As I understand it, I should use the postStratify() function in the survey package for this task, but I must admit that I am quite lost when it comes to the required syntax.

I have tried to use marginal distributions and the rake() function, this is quite straight-forward: http://www.r-bloggers.com/survey-computing-your-own-post-stratification-weights-in-r/

But how do I incorporate all my joint distributions into the postStratify() function? The manual only lists a simple example and is a bit vague, to me at least. I have full joint distributions for 5 different variables.

Moreover, if want to use my weighted result with a function that does not take weight as an input, how should I go about and "duplicate" my rows in the final data? When I tried the rake() function I could only trim weights to an interval with a min value of 0.87. I was thinking I could round all weights to integers.

• some syntax examples github.com/ajdamico/usgsd/… Commented Mar 10, 2015 at 1:07
• Please demonstrate your syntax. Commented Aug 11, 2016 at 13:00
• I just answered a similar question on postStratify() syntax here. Commented Aug 15, 2016 at 18:50

I made an example for using post-stratification as below. I find that in the 'strata' argument, if specify as a formula with two or more variables, it implies that the post-stratification is done for their joint distribution. (Initially, I thought I need to add an interaction term in the formula, but it turned out to be unnecessary!)

library(survey)

data(api)
summary(apiclus1)

# original design
# suppose 'pw' is sampling weight
dclus1<-svydesign(id=~dnum, weights=~pw, data=apiclus1, fpc=~fpc)

### post-stratify on only one categorical variable ###
# on school type
pop.types <- xtabs(~stype, data=apipop)
dclus1p<-postStratify(dclus1, ~stype, pop.types)

# compare sampling weights with post-stratified weights
summary( attr(dclus1p$$postStrata[[1]], "oldweights" ) ) summary( attr(dclus1p$$postStrata[[1]], "weights" ) )

### post-stratify on joint distribution of two categorical variables ###
# school type and sch.wide

apiclus1$$stype.sch.wide <- interaction( apiclus1$$stype, apiclus1$$sch.wide ) apipop$$stype.sch.wide <- interaction( apipop$$stype, apipop$$sch.wide )
pop.types <- xtabs(~stype.sch.wide, data=apipop)

dclus1<-svydesign(id=~dnum, weights=~pw, data=apiclus1, fpc=~fpc)

# first method
dclus2p<-postStratify(dclus1, ~stype.sch.wide, pop.types)
summary(weights(dclus2p))

# second method
pop.types <- xtabs(~ stype + sch.wide, data=apipop )
dclus3p<-postStratify(dclus1, ~stype + sch.wide, pop.types)
summary(weights(dclus3p))


If you are having syntax trouble calculating post stratification weights and trimming I can recommend using www.spinnakerresearch.nl. No syntax needed. I believe you can trim max weights. However rounding weights to integers does not make sense as you will need to weight some cases down. Meaning a weight factor between 0 and 1.

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