[R] Confidence interval calculation in prop.test

[Douglas Bates]

On 10/30/06, Richard Johnston <rmjohnston at mac.com> wrote:
> The confidence interval calculation in prop.test appears to be
> incorrect when alternative="greater".  The upper limit is always set
> to 1.000.  Am I missing something?

Like the t.test function the prop.test function returns a one-sided
interval when the alternative is "greater" or "less".  The output
indicates that the 95% lower confidence bound on the proportion is
0.0369.  If you choose alternative = "less" you will get a 95% upper
confidence bound (and the lower bound will always be given as 0).
Only when you use the default value, "two-sided", for the alternative
parameter will you get both the upper and lower bounds calculated.

>  > total=c(250,250)
>  > success=c(55,31)
>  > prop.test(success,total,alternative="greater",correct=TRUE)
>
>         2-sample test for equality of proportions with continuity correction
>
> data:  success out of total
> X-squared = 7.4289, df = 1, p-value = 0.003209
> alternative hypothesis: greater
> 95 percent confidence interval:
> 0.03693065 1.00000000
> sample estimates:
> prop 1 prop 2
> 0.220  0.124
>
>
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