Jamie <[email protected]> asks why the -svy- prefix with the -subpop()- option
dropped observations from the estimation sample:
> I am trying to select a subpopulation for use with svy: mlogit.
>
> The variable which specifies the subpopulation has 1130 observations
> with a value of 1, 1792 0s and no missing data. However, when I run
> the syntax (below), only 1099 observations are included in the analysis
>
> xi: svy, subpop(varname): x y1 y2, rrr
>
> Searching on the archives here brought me to a post about 'losing
> observations'
>
> Subject st: Why does one command "lose" observations compared to
> another command?
> Date Sun, 22 Jan 2006 11:56:50 -0500 "
>
> This problem seemed similar to the one I have and the answers
> suggested that it might be something to do with a weights variable
> having zeros on it.
>
> I have checked my probability weight variable which is invoked earlier
> in the syntax by svyset, and it contains 121 0s, which doesn't quite
> explain the 131 missing observations.
>
> When I specify the brr option to include both the probability weight
> and bootstrap weights (see below), all 1130 observations ARE included
> in the analysis
>
> xi: svy brr, subpop(varname): x y1 y2, rrr
>
> I would really appreciate any help.
The -svy- prefix drops strata that do not contain observations that overlap
with the specified -subpop()- option. In addition, observations where the
sampling weight variable is zero are also considered outside the
subpopulation.
Suppose stratum 1 is identified in the dataset with the following 10
observations:
obs pweight subpop
1 0 1
2 0 1
3 0 1
4 0 1
5 33 0
6 19 0
7 5 0
8 21 0
9 8 0
10 30 0
Then stratum 1 is going to be omitted by -svy- because none of it's
observations overlap with the subpop, 0 values in 'pweight' overrule the value
of the 'subpop' variable.
Since the 28aug2006 executable update in Stata 9, the -svy brr- prefix has
been using the same logic for omitting strata.
--Jeff
[email protected]
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