Zurab Sajaia <[email protected]> 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
[email protected]
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