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RE: st: Strange behavior or bug? - svy : total returns different results


From   Zurab Sajaia <zsajaia@hotmail.com>
To   statalist <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Strange behavior or bug? - svy : total returns different results
Date   Fri, 28 Aug 2009 17:24:44 -0400

Jeff,
 
Thanks a lot for the explanation, although I wouldn't say that I got the point - if zero weight-observations are interpreted as "out of interest" why are thy kept in the sample? and if they are, then why aren't they dropped as containing missing values in the variable of interest? (mv)
 
zurab

----------------------------------------
> From: jpitblado@stata.com
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Strange behavior or bug? - svy : total returns different results
> Date: Fri, 28 Aug 2009 14:51:36 -0500
>
> Zurab Sajaia is using -svy: total- with a variable
> containing missing values and zero sampling weights:
>
>> I've encountered a strange behavior of -svy- commands and was wandering
>> whether this is a bug or I'm doing something wrong, any thought would be
>> greatly appreciated.
>>
>> the setup:
>>
>> sysuse auto, clear
>> svyset [pw = weight]
>>
>> generate mv = .
>>
>> svy : total mv
>>
>> all versions (I have) of Stata (9-11) return -no observation- error after
>> this.
>>
>>
>> but if we change weights:
>>
>> replace weight = 0 in 1/5 // there might be other explanation but I found that this
>> // error happens only if I have some observations with zero weights
>>
>> then Stata 9 still shows error 2000 but both 10 and 11 versions return an
>> output with "(sum of weights equals zero)" message on the screen and e(b)
>> matrix equal to 0.
>
> In Stata 10, we fixed how -svy: total- (and all other -svy- estimation
> commands) handle missing values in observations outside the subpopulation.
> We no longer mark out missing values for observations that are outside the
> subpopulation.
>
> Zurab had constructed a degenerate case for subpopulation estimation, even
> though the -subpop()- and -over()- options were not specified.
>
> By setting the sampling weight to 0 in the first 5 observations, Zurab has
> indicated to -svy- that those observations are out of the subpop of interest.
> The missing values in 'mv' cause -svy: total- to drop all the remaining
> observations. Thus there are 5 observations outside the subpop and 0 inside.
>
> The estimation sample contains 5 observations, all of which contain zero
> valued sampling weights, and -svy: total- reports this in the estimation
> table.
>
> Stata 9 did not receive this fix, thus it drops all observations containing
> missing values regardless of the values in the sampling weight variable.
>
> --Jeff
> jpitblado@stata.com
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