[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From (Jeff Pitblado, StataCorp LP)
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

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

Stata 9 did not receive this fix, thus it drops all observations containing
missing values regardless of the values in the sampling weight variable.

*   For searches and help try:

© Copyright 1996–2017 StataCorp LLC   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index