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Re: st: subpop and the mysterious sample size


From   ROBERT BOZICK <rbozick1@jhem.jhu.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: subpop and the mysterious sample size
Date   Fri, 19 May 2006 10:40:09 -0400

That makes sense, but there is another hitch....

The reason for needing the correct sample size is that I need to compute the population standard deviation.  I have been using the command suggested in http://www.stata.com/support/faqs/stat/supweight.html

The post estimation command after I use the svymean is:
di sqrt(e(N) * el(e(V_srs),1,1))

Additionally, I will be using some by commands as well.    For example:

svymean var1, subpop(samp) by(sex)

and then I want to compute the population standard deviation for var1 for both categories of sex.  Without the sample size, I cannot get the correct standard deviation using the suggested post estimation command.



----- Original Message -----
From: jpitblado@stata.com (Jeff Pitblado, StataCorp LP)
Date: Friday, May 19, 2006 10:14 am
Subject: Re: st: subpop and the mysterious sample size

> ROBERT BOZICK <rbozick1@jhem.jhu.edu> is concerned abou the 
> 'Number of obs'
> output in the header of -svymean- (replaced by -svy: mean- in 
> Stata 9):
> 
> > 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> ---------------------------------------------------------
> ---------------------
> 
> In Stata 8, unlike -svyregress-, -svymean- does not display the 
> subpopulationsample size.  However Robert could use -svyregress- 
> to compute a subpopulation
> mean
> 
> 	. svyregress var1, subpop(samp)
> 
> and have the subpopulation size and its sample size get reported 
> in the
> header.  Note that the subpopulation mean estimate and its 
> standard error are
> the same whether you use -svymean- or -svyregress-.
> 
> In Stata 9, all the -svy- estimation commands have a unified 
> header, and this
> header includes the subpopulation size and its sample size too.  
> The Stata 9
> syntax, using Robert's example is
> 
> 	. svy, subpop(samp) : mean var1
> 
> --Jeff
> jpitblado@stata.com
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