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

[Stas Kolenikov <skolenik@gmail.com>]

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.

-- 
Stas Kolenikov, also found at http://stas.kolenikov.name
Small print: I use this email account for mailing lists only.

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/