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st: subpop and the mysterious sample size


From   ROBERT BOZICK <rbozick1@jhem.jhu.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: subpop and the mysterious sample size
Date   Fri, 19 May 2006 08:51:13 -0400

Hi Stata-listers, 

I am working on a project where I need to use survey commans to estimate the standard error correctly --- my sample uses a stratified cluster design.   I created a variable called samp to indicate the analytic sample.  When samp = 1, then the respondent will be included in the analysis; when samp = 0 then the respondent will not be included in the analysis.  

The frequency of samp is shown below: 

tab samp 

       samp |      Freq.     Percent        Cum. 
------------+----------------------------------- 
          0 |      6,917       42.25       42.25 
          1 |      9,456       57.75      100.00 
------------+----------------------------------- 
      Total |     16,373      100.00 

As you can see, there should be 9,456 in my analysis.
When I use the svy commands to estimate means for my analytic sample using the subpop command,  the output reports that there are 15,548 used in the analysis.  Intuitively, that cannot be correct.  Does anyone know what is going on here?  How can I fix this so that it reports 9,456 instead of 15,548?  Thanks!

svymean var1, subpop(samp)  

Note: 11 strata omitted because they contain no subpopulation members

Survey mean estimation
pweight:  f1pnlwt                                 Number of obs    =     15548
Strata:   strat_id                                Number of strata =       350
PSU:      psu                                     Number of PSUs   =       729
Subpop.:  samp==1                                 Population size  = 3312561.5
------------------------------------------------------------------------------
    Mean |   Estimate    Std. Err.   [95% Conf. Interval]        Deff
---------+--------------------------------------------------------------------
    var1 |   46.72176    .2878163    46.15584    47.28768    4.473885
------------------------------------------------------------------------------

Robert
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