Re: st: Number of Obs with svy , suppop()

[Michael Norman Mitchell <Michael.Norman.Mitchell@gmail.com>]

Dear Phil and Stas

  Thank you very kindly for the extra information. Using my ordinary 
intuition, I just could not fathom how imposing a restriction on the 
sample (via subpop()) could lead to an increase in the "number of obs". 
Your additional explanation helped quite a bit.
  To me, this sounds like an excellent Stata FAQ. I am going to write 
to Stata and make such a suggestion.
Many thanks,

Michael N. Mitchell
See the Stata tidbit of the week at...
http://www.MichaelNormanMitchell.com

On 2010-03-19 1.20 PM, Stas Kolenikov wrote:
> On Fri, Mar 19, 2010 at 3:17 AM, Michael Norman Mitchell  wrote:
>    
> > Dear Phil
> >
> >   Thank you for your reply... I am still struggling to solidly understand
> > this. Perhaps I have a more fundamental question. What is the formula for
> > the "Number of obs" in the context of the -svy- commands. It sounds like, in
> > the absence of the -subpop()- option, it is the number of observations with
> > non-missing values on the tabulated variable. And, in the presence of the
> > -subpop()- option it is the total number of observations minus the number of
> > observations that meet the -subpop()- option and are missing on the
> > tabulated variable. Am I on the right track here?
> >      
> This is a complicated interplay between -markout-s of the survey
> design variables, survey subpopulation, and that of the very command
> to be called. I guess in this case what happened was:
>
> 1. -tab- marked out observations for which either race or gender were
> missing, resulting in 4000 observations.
>
> 2. next, -subpop- marked out the observations with sex==1.
>
> 3. Finally, -svy- looked at these markings, and decided that the total
> # of observations must be the number used in estimation in the
> subpopulation (which turns out to be the intersection of what -tab-
> and -subpop- has identified as relevant observations, 1904 males with
> non-missing race), plus the number that was not marked out by either
> command (2133 females, regardless of their race variable value). That
> meant all individuals with sex==2, including those with missing race
> information.
>
> Frankly, I don't know what the "correct" behavior should be. I guess
> it is extremely difficult for a prefix command like -svy- to figure
> out what's going on within the prefixed command (like -tab-). The
> biggest culprit was -tab- which carelessly excluded some observations
> from its -e(sample)- and did not know that -svy- would need to count
> all these extra observations that -tab- dropped (essentially). What
> Phil gave with an "extended" subpop specification is certainly a good
> working solution, but it demands substantial discipline from the
> user/analyst. It also explicitly says that the part of the population
> to whom the result can be generalized are the people who do not hide
> their race.
>
>    
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