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st: CI for a difference in proportions


From   Antoine Terracol <terracol@univ-paris1.fr>
To   statalist@hsphsun2.harvard.edu
Subject   st: CI for a difference in proportions
Date   Wed, 27 Feb 2008 20:18:32 +0100

Dear _all,

This might be a silly question, but I currently have no access to 
Stata's user manuals or stats textbook.

I have data on two independant samples (of different sizes), each 
observed over 20 periods. I wish to draw a graph representing, for each 
period, the difference in the proportion of some event between the two 
groups, together with the 95% CI for this difference.

I first wanted to use -prtest- for each period, store/compute the 
differences and CI bonds in new variables, and graph them against periods.

However, I am a bit confused over which standard error to use (and how 
to get them) when computing the CI bounds.

-prtest- gives two standard errors, the one under H0 can be easily 
computed from stored results r(P_1), r(P_2) and r(z). The standard 
errors for each proportion (se1 and se2) are not stored, and I do not 
know if the fact that my calculation of sqrt(se1^2 + se2^2) differs from 
the reported standard error for the difference (not under H0) is caused 
by rounding errors or if my formula is incorrect...

I thought of using -proportion- to get the standard errors as stored 
results, but it actually gives standard errors for each proportion that 
differ from -prtest-, but are identical to the ones computed by -mean-.

Any advice would be appreciated.

Antoine



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