[R] ?Accuracy of prop.test

Greg Snow Greg.Snow at imail.org
Mon Jul 18 22:51:23 CEST 2011

```Others have explained why R gives a different answer based on a different approximation, but if you want to get the same answer as the book/minitab/... for your own understanding (or so the grader doesn't get confused by superior answers, or other reasons) here is one way to do it:

> x <- c( rep(1,202), rep(0, 1010-202) )
> p <- 202/1010
> sd <- sqrt( p*(1-p) )
>
> library(TeachingDemos)
> z.test( x, 0.5, sd )

One Sample z-test

data:  x
z = -23.8354, n = 1010.000, Std. Dev. = 0.400, Std. Dev. of the sample
mean = 0.013, p-value < 2.2e-16
alternative hypothesis: true mean is not equal to 0.5
95 percent confidence interval:
0.1753312 0.2246688
sample estimates:
mean of x
0.2

Which matches the others you reported below.

--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Jack Sofsky
> Sent: Sunday, July 17, 2011 10:27 AM
> To: r-help at r-project.org
> Subject: [R] ?Accuracy of prop.test
>
> I have just joined this list (and just started using R), so please
> excuse any etiquette breaches as I do not yet have a feel for how the
> list operates.
>
> I am in the process of teaching myself statistics using R as my utility
> as my ultimate goals cannot be satisfied by Excel or any of the plug-
> ins
> I could afford.
>
> I am currently looking at chap12 page 552 of Weiss's Introductory
> Statistics 9th edition.  Example 12.5 demonstrates using "Technology"
> to
> obtain a One-Proportion z-Interval.
>
> n=202
> x=1010
> confidence interval = .95.
>
> 0.175331, .224669
> .17533, .22467
> Answer given by Weiss's Excel Plug-in
> 0.175 < p < 0.225
>
> Here is what I got with R
> prop.test(202,1010,correct="FALSE")
>
>      1-sample proportions test without continuity correction
>
> data:  202 out of 1010, null probability 0.5
> X-squared = 363.6, df = 1, p-value < 2.2e-16
> alternative hypothesis: true p is not equal to 0.5
> 95 percent confidence interval:
>   0.1764885 0.2257849
> sample estimates:
>    p
> 0.2
>
> I'm also getting slight differences in the answers for exercises and
> find this disconcerting.
>
> Why are these differences present  (or am I doing something wrong)?
>
> Jack
>
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