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variables for svyset in Stata (Bangladesh 2011) [message #3566] 
Fri, 09 January 2015 13:31 
kmorris
Messages: 5 Registered: December 2014

Member 


Hello,
I am currently trying to analyze csection by wealth quintile (lower 2 and upper 3 as their own groupings).
I am using Stata 12, and when trying to set svyset for the data, I have been encountering an issue with the strata that is affecting my results in analysis.
gen birth5=0
replace birth5=1 if v208>0
label var birth5 "have a live birth in past 5 years"
label define yesno 0 "no" 1 "yes"
label values birth5 yesno
keep if birth5 == 1
gen wt=v005/1000000
gen csect = . if m17 == .
replace csect = 1 if m17 == 1
replace csect = 0 if m17 == 0
label variable csect "CSection"
label define csect 0 "no" 1 "yes"
label value csect csect
gen psu=v021
gen strata=v023
**note, I generated the weight earlier, above)**
svyset psu [pweight=wt], strata(strata)
**now analyzing using poisson for the lower 2 wealth quintiles**
svy: poisson csect v190 if v190<3
And this is where I run into errors, see results as written below
(running poisson on estimation sample)
Survey: Poisson regression
Number of strata = 19 Number of obs = 3645
Number of PSUs = 499 Population size = 3867.0893
Design df = 480
F( 0, 480) = .
Prob > F = .
 
 Linearized
csect  Coef. Std. Err. t P>t [95% Conf. Interval]
+ 
v190  1.011505 . . . . .
_cons  4.721347 . . . . .
 
Note: missing standard errors because of stratum with single sampling unit.
Did I do something wrong with the sampling unit? Is this error unique to this dataset or will I have to change my syntax?
Thank you!
Kate Morris







Re: variables for svyset in Stata (Bangladesh 2011) [message #3739 is a reply to message #3738] 
Tue, 03 February 2015 22:58 
mmrUMICH
Messages: 21 Registered: February 2015 Location: A2, MI

Member 


Thank you, Trevor.
I will cut and paste some lines whenever requires from original message and try to clarify concretely the reasons behind that error/warnings and its possible solution:
**now analyzing using poisson for the lower 2 wealth quintiles**
svy: poisson csect v190 if v190<3
The above svy: command is not recommended [and not correctly handle the domain concept] as "if v190<3" in svy: subsets the data (i.e., also deleting design information [aside: full sample design information is important for correctly calculating the sampling errors]) prior to run the poisson regression.
This svy: command uses such a subset data and as a result analysis sample (i.e. "estimation sample" in Stata wording) lacks one strata and 101 PSUs (see below output (cut and paste):
start
(running poisson on estimation sample)
Survey: Poisson regression
Number of strata = 19 Number of obs = 3645
Number of PSUs = 499 Population size = 3867.0893
Design df = 480
 end 
We have to create a variable, say: mydomain = 1 if v190 < 3, otherwise, mydomain = 0, then use svy command:
svy, subpop(mydomain): poisson csect v190
I hope this run will not encounter such issue and also does not require singleunit(centered)* svyset option. And the output will show the same # of obs and population size, but changed others stats such as # of strata, PSUs and degrees of freedom (df).
I verified that strata 5 and 11 have 5 and 23 PSUs respectively; so this svy, subpop(): that form "analytic" domain/subpopulation/subgroup will not be an issue of singletonstrata from full sample data.
*note that singleunit(method) is kind of practically recommended for "analytic" subgroup and/or subclass analysis which sometimes encounter singletonstrata. This specification also appropriately calculates the degrees of freedom, which is prim important for statistical inferences, e.g, confidence intervals and pvalues estimation.
Thank you all again.
Moshiur Rahman




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