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Re: st: Confidence Interval for Proportion

From (Jeff Pitblado, StataCorp LP)
Subject   Re: st: Confidence Interval for Proportion
Date   Tue, 11 Mar 2008 13:08:17 -0500

Martin Weiss <> is using the -proportion- command
and has a question about how standard errors are computed:

> Dear Statalisters,
> try this in Stata:
> ************************
> sysuse auto, clear
> proportion rep78
> matrix define A=e(b)
> matrix define B=e(V)
> count if rep78!=.
> *Upper/Lower Bound for proportion of "1"
> di A[1,1]+invnormal(1-0.05/2)*sqrt(A[1,1]*(1-A[1,1])/`r(N)')
> di A[1,1]-invnormal(1-0.05/2)*sqrt(A[1,1]*(1-A[1,1])/`r(N)')
> *Standard Error for "1"
> *Mistake obviously there...
> di sqrt(A[1,1]*(1-A[1,1])/`r(N)')
> ************************
> Then let me know: why do I not hit the correct CI for the proportion of "1"
> in the repair record? Something`s wrong with the standard error, I do not
> know what, though...

Using Martin's example Stata code, -proportion- effectively computes the
standard error via


This is explained (rather tersely, I'll admit) in the 'Methods and Formulas'
section of -[R] proportion-.

	"Proportions are means of indicator variables; see -[R] mean-."

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