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Re: st: tabulate, summarize() for svy data


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: tabulate, summarize() for svy data
Date   Fri, 19 Mar 2010 09:30:25 -0700 (PDT)

Thank you Steve.
Just to clarify. You multiplied the variance of the mean by the sample size of the population, to get the Standard Deviation and not the Standard error, correct?

If yes, do I compute the lower limit for the 95% CI using the SE as:

local ll= el(mm,1,`i') - sqrt(el(VV,`i',`i'))*invttail(e(df_r),(5)/200) 

assuming a t-distribution?

Ricardo
  
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK


--- On Fri, 3/19/10, Steve Samuels <sjsamuels@gmail.com> wrote:

> From: Steve Samuels <sjsamuels@gmail.com>
> Subject: Re: st: tabulate, summarize() for svy data
> To: statalist@hsphsun2.harvard.edu
> Date: Friday, March 19, 2010, 10:41 AM
> On Fri, Mar 19, 2010 at 9:34 AM,
> Ricardo Ovaldia <ovaldia@yahoo.com>
> wrote:
> > Steve,
> >
> > Why does your program use e(V_srssub) for the
> variance, instead of e(V) or e(V_srs).
> I made a serious error in the code. The revision is below.
> 
> To answer your question:
> 
> the -over()- optioon triggers variance estimates
> appropriate for
> subpopulations, just as  a  - survey, subpop()-
> prefix would have
> done.
> 
> e(V) is the  estimated variance of for each mean
> (square of standard error)
> e(V_srs) is the  estimated variance for each mean, if
> there had been
> simple random sampling of the entire population
> e(V_srssub) is the estimated variance for each mean if
> there had been
> simple random sampling within each subpopulation. 
> (The full
> designation is "e(V_srssubpop)")
> 
> See the section on Design Effects with subpopulations on
> page 35 of
> the Version 11 Survey Manual.
> 
> To estimate the variance of the observations in each
> subpopulation,
> it's necessary to multiply  the variance of the mean
> by the sample
> size of the population.  I forgot to do that. 
> Also, I changed the
> names of the matrices to avoid potential ambiguities. Here
> is the
> corrected version.
> 
> 
> **************************CODE
> BEGINS**************************
> sysuse auto, clear
> svyset _n
> 
> svy: mean mpg, over(foreign)
> matrix define mm = e(b)  //mean (already shown)
> matrix define VV = e(V_srssub) // variances for
> subpopulations
> matrix define nn = e(_N)   //sample sizes
> 
> local kk = colsof(V)
> forvalues i = 1/`kk'{
> di `i'  " "  el(mm,1,`i')  " "
> sqrt(el(nn,1,`i')*el(VV,`i',`i'))  " "
>  el(nn,1,`i')
> }
> ***************************CODE
> ENDS***************************
> 
> 
> Steven Samuels
> sjsamuels@gmail.com
> 18 Cantine's Island
> Saugerties NY 12477
> USA
> 845-246-0774
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