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Re: st: svyset for a multistage sampling design


From   "Moises Rosas (w)" <morofe210-statalist@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: svyset for a multistage sampling design
Date   Tue, 25 Aug 2009 12:48:43 -0700 (PDT)

Dear Tomeka,
 
The first thing is that it's very unlikely that schools are stratum. You have to look carefully for the stratum variable. 
 
Second, you don't need to put all the design information on the svyset command (take a look at "A Note on Variance Estimation" in http://support.sas.com/rnd/app/da/new/dasurvey.html: "For a multistage sample design, the variance estimator depends only on the first stage of the sample design. Thus, the required input includes only first stage cluster (PSU) and first-stage stratum identification. You do not need to input design information about any additional stages of sampling. This variance estimation method assumes that the first stage sampling fraction is small or that the first-stage sample is drawn with replacement, as it often is in practice. "
 
Third, look for the pweight for the entire design (not just for the first stage). It’s the inverse of the product from multiplying all sampling probabilities for all stages (it has to be there, in your data file. If not, you do the math). 
 
Finally, with that info, all you have to do is to svyset something like this:
 
svyset county [pweight=pwforalldesign], strata (yourthruestratumvariable)
 
Good luck!

Moises




________________________________
From: Tomeka Davis <soctmd@langate.gsu.edu>
To: statalist@hsphsun2.harvard.edu
Sent: Friday, August 21, 2009 8:14:20 AM
Subject: Re: st: svyset for a multistage sampling design

Thanks.  I have one other question.  In response to the question about needing an fpc in STATA, a member of the data collection team responded by saying:

"STATA can incorporate a finite population correction (fpc), but does not require one. In the data, primary sampling units (PSUs) were selected using the Durbin method which allows variances to be estimated as if selection was done "with replacement." When sampling is done with replacement, an fpc is unnecessary."

Is there a way to specify this in STATA without the strata being ignored?

Thank you for your help.


>>> "Michael I. Lichter" <MLichter@Buffalo.EDU> 8/20/2009 7:31 PM >>>
Tomeka,

1. If you don't supply a finite population correction option for the 
first stage [svyset psu, fpc()], svyset will ignore your second stage.
2. What are your school strata and how many of the strata have only only 
psu in them -- and why? It doesn't look to me like school is a 
stratifying variable -- it's your second stage sampling unit which will 
be ignored without the FPC.

-ml

Tomeka Davis wrote:
> I am trying to use the set of svy commands to analyze data collected using a multi-stage design.  The first stage of sampling involved counties that were selected proportional to their size (N~100).  Next, schools were sampled (N~400), and then students were randomly sampled within schools.
>  
> In this context, the county is the psu and the school is the strata. I am not sure how strata with one sampling unit were treated.  Because I must have this option in  STATA, I choose the certainty option since it contributes nothing to the standard error.  Without the option, the program does not calculate standard errors.
>  
> My syntax looks like :  svyset county [pweight=studentweight], strata (school) ||_n, singleunit (certainty)
>  
> The problem is that I am not clear if STATA is using all of the stages.  After I enter the command, I get a message that says:
>  
> "Note:  stage 1 is sampled with replacement; all further stages will be ignored".
>  
> I would appreciate any comments.
>
>
> *
> *  For searches and help try:
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> *  http://www.ats.ucla.edu/stat/stata/ 
>  

-- 
Michael I. Lichter, Ph.D. <mlichter@buffalo.edu>
Research Assistant Professor & NRSA Fellow
UB Department of Family Medicine / Primary Care Research Institute
UB Clinical Center, 462 Grider Street, Buffalo, NY 14215
Office: CC 126 / Phone: 716-898-4751 / FAX: 716-898-3536

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