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st: Different confidence intervals from proportions and tabulates (also in survey)


From   "Jason Ferris" <J.Ferris@latrobe.edu.au>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Different confidence intervals from proportions and tabulates (also in survey)
Date   Mon, 30 Oct 2006 09:20:11 +1100

Hi. I have been running survey proportions and observing results with
negative confidence intervals (which doesn't make sense).  When I use
survey tab (with column percent, se and ci) I get the same point
estimates and standard errors but different 95% confidence intervals.  I
assume this is an issue with the proportion calculations using "Binomial
Wald" for confidence intervals.

 

I checked the survey manual and have not been able to find why:

Paste the following command to see my dilemma:
webuse nhanes2b, clear
svy: proportion race
svy: tab race, ci se

 

The results show the same point estimates and standard errors (with
rounding) but different CI's.  As mentioned, for my data, I get some
negative CI's for svy: proportions commands but not for the svy:
tabulate commands. My ultimate concern is being able to automatically
extract the CORRECT estimates to excel (from using matrix e(b) and e(V)
- and calculating 95% CI from square-root of e(V) *1.96).

I am using the latest version of Stata 9.2, on Windows XP.


Jason Ferris

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