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Post-stratification for DHS data [message #11194] Wed, 16 November 2016 22:36 Go to next message
sd535 is currently offline  sd535
Messages: 8
Registered: March 2013
Location: Wollongong, NSW, Australi...
Member

Hi,
I wish to post-stratify the Bangladesh DHS data at district-level according to the total number of children per district extracted from the recent census.
I use the following code for post-stratification. However, I didn't get any change after post-stratification. Am I doing any wrong?

Preliminary Design:

DHSdesign <- svydesign(id = child.data.HAZ$V001, strata=child.data.HAZ$V023, weights = child.data.HAZ$V005/1000000, data=child.data.HAZ)

ps.weights <-
data.frame(
CODIST = c (PostStrata.Wt.Cal$dist.id) , # District ID (relates to CODIST )
Freq = c( PostStrata.Wt.Cal$children.census ) # Number of children under 5
)

DHSdesign.Post.Strata <-
postStratify(
DHSdesign ,
strata = ~CODIST ,
population = ps.weights
)

DE.MEAN.HAZ.District<-svyby(~HW70, ~CODIST, DHSdesign, svymean)
PS.DE.MEAN.HAZ.District<-svyby(~HW70, ~CODIST, DHSdesign.Post.Strata, svymean)
SE.DE.MEAN.HAZ.District<-svyby(~HW70, ~CODIST, DHSdesign, svymean)[,3]*100
PS.SE.DE.MEAN.HAZ.District<-svyby(~HW70, ~CODIST, DHSdesign.Post.Strata, svymean)[,3]*100

> summary(DE.MEAN.HAZ.District)
Min. 1st Qu. Median Mean 3rd Qu. Max.
-241.6 -185.3 -162.2 -167.6 -146.8 -112.0
> summary(PS.DE.MEAN.HAZ.District)
Min. 1st Qu. Median Mean 3rd Qu. Max.
-241.6 -185.3 -162.2 -167.6 -146.8 -112.0

> summary(SE.DE.MEAN.HAZ.District)
Min. 1st Qu. Median Mean 3rd Qu. Max.
0.0 935.5 1326.0 1298.0 1612.0 2536.0
> summary(PS.SE.DE.MEAN.HAZ.District)
Min. 1st Qu. Median Mean 3rd Qu. Max.
0.0 935.5 1326.0 1298.0 1612.0 2536.0

There is no difference. And the most im[portant problem is that the SE for one area is zero.

Can you please inform me where I am doing the wrong.


Sumonkanti Das
Re: Post-stratification for DHS data [message #11202 is a reply to message #11194] Thu, 17 November 2016 12:19 Go to previous messageGo to next message
Bridgette-DHS is currently offline  Bridgette-DHS
Messages: 2552
Registered: February 2013
Senior Member
Following is a response from Senior DHS Stata Specialist, Tom Pullum:


Quote:
I'm sorry, but I don't use SPSS, and I can't figure out what you did.

I would calculate the total number of weighted cases that you want to have in each district. I think of these as "target" totals. They are obtained by multiplying the total number of cases (unweighted) in your file by the proportions that are in each district in the census. You then multiply the weight (v005) in each district by the ratio of the target total to the current weighted number of cases in the district.

I will illustrate how you would do this in Stata to obtain a uniform distribution of weighted cases across the seven region (v024):

set more off
use e:\DHS\DHS_data\KR_files\BDKR70FL.dta, clear
sort v024
save e:\DHS\DHS_data\scratch\BDtemp.dta, replace

keep v024 v005

gen wtd_n=v005/1000000
gen unwtd_n=1
collapse (sum) *wtd_n, by(v024)
list, table clean

* adjust v005 so that the total weight will be the same in each region

summarize unwtd_n
scalar stotal=r(sum)
scalar list stotal
gen target=stotal/7
gen v005_factor=target/wtd_n
list, table clean
keep v005_factor v024

sort v024
merge v024 using e:\DHS\DHS_data\scratch\BDtemp.dta
tab _merge

gen v005_rewtd=round(v005*v005_factor)


*check that the new distribution matches the targets
gen wtd_n=v005/1000000
gen rewtd_n=v005_rewtd/1000000
gen unwtd_n=1
collapse (sum) *wtd_n, by(v024)
list, table clean
Re: Post-stratification for DHS data [message #11394 is a reply to message #11202] Sun, 11 December 2016 08:35 Go to previous messageGo to next message
sd535 is currently offline  sd535
Messages: 8
Registered: March 2013
Location: Wollongong, NSW, Australi...
Member

Dear Tom Pullum,
We followed your instruction and get results exactly same with/without post-stratification. For your convenience, we are sending you the data with stata code and a theoritical statement in a pdf file.
Can you please instruct us whether we are in right track to provide some statistics (with SE) at district-level?

Regards,
Sumon


Sumonkanti Das
Re: Post-stratification for DHS data [message #11409 is a reply to message #11394] Tue, 13 December 2016 15:36 Go to previous message
Bridgette-DHS is currently offline  Bridgette-DHS
Messages: 2552
Registered: February 2013
Senior Member
Following is a response from DHS Senior Research Associate, Shireen Assaf:

Dear Sumon,

Following Tom's instructions, you forgot to perform one step which is important to have the total weight same in each district (i.e. divide by the number of districts which is 64). I performed this step in the attached do file, and when I use the svy with the different weights I obtain different standard errors.

Please change the paths in the do file back to your paths.

Thank you.

Best regards,

Shireen Assaf
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