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Re: st: Different results from svy: tab vs. svy: prop


From   jpitblado@stata.com (Jeff Pitblado, StataCorp LP)
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Different results from svy: tab vs. svy: prop
Date   Tue, 24 Oct 2006 13:24:16 -0500

Arnold Levinson <Arnold.Levinson@UCHSC.edu> is comparing -svy: proportion- and
-svy: tabulate- estimation results using the -stdize()- option:

> I'm comparing tobacco survey data for two years. I want to standardize
> estimates for the later year (2005) using age-sex-ethnicity distribution
> of the earlier year (2001). When I run the estimates separately using -
> svy: prop - I get the expected results: std-ized and non-std-ized
> estimates are essentially the same for 2001 but different for 2005.
> However, when I run the contrast directly using - svy: tab - the
> std-ized estimates change by 0.1% for 2001 (the reference year for
> std-ized weights) but don't change for 2005. Can someone explain why
> this happens? Here's the output.

> (output omitted)

Michael Blasnik already pointed out that the differences could be due to a
rescaling of the standardization weights when one or more standard strata are
not represented in the 2005 data.  Use -tabulate- to see if there are any
unrepresented standard strata in the 2005 data.

Another difference is that the -if- condition used with -svy: proportion-
results in proportions relative to the restricted sample, whereas the
proportions reported by -svy: tabulate, column- are column proportions
computed from the estimated cell proportions of the twoway table.  While the
unstandardized calculations result in the same values, this is not guaranteed
to be the case for the standardized calculations.

--Jeff
jpitblado@stata.com
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