R help - Apr 2013 - prop.test vs hand calculated confidence interval

Hi,

This code:

n=40
x=17
phat=x/n
SE=sqrt(phat*(1-phat)/n)
zstar=qnorm(0.995)
E=zstar*SE
phat+c(-E,E)

Gives this result:

[1] 0.2236668 0.6263332

The TI Graphing calculator gives the same result.

Whereas this test:

prop.test(x,n,conf.level=0.99,correct=FALSE)

Give this result:

0.2489036 0.6224374 

I'm wondering why there is a difference.

D.



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Sent from the R help mailing list archive at Nabble.com.

One of the joys of R is that it's open source: you can read the code
for prop.test yourself and see what's happening.

In this case, simply typing
prop.test
at the command line will provide it, although without comments.

Sarah



On Wed, Apr 3, 2013 at 6:10 PM, David Arnold <dwarnold45 at
suddenlink.net> wrote:
> Hi,
>
> This code:
>
> n=40
> x=17
> phat=x/n
> SE=sqrt(phat*(1-phat)/n)
> zstar=qnorm(0.995)
> E=zstar*SE
> phat+c(-E,E)
>
> Gives this result:
>
> [1] 0.2236668 0.6263332
>
> The TI Graphing calculator gives the same result.
>
> Whereas this test:
>
> prop.test(x,n,conf.level=0.99,correct=FALSE)
>
> Give this result:
>
> 0.2489036 0.6224374
>
> I'm wondering why there is a difference.
>
> D.
>
>
--
Sarah Goslee
http://www.functionaldiversity.org