by definition, the one tailed p-value has to be <= 0.5 so there is still something wrong with your OpenEpi calc. Most likely it's calculating the
2 tailed p-value  and then mistakenly multiplying by 2. For example:

A) Suppose you are testing

                               Ho: u = u_0
                               H1 u > u_0

and your t-stat was -0.3 Then prob( T > t_0) = 0.62 so your pvalue would be 0.62.


B) Instead, suppose you are testing

                               Ho: u = u_0
                               H1 u != u_0

and your t-stat was -0.3. Then, one calculates, prob(T < t_0 = -.3) = .31 and then multiplies by 2 ( because the test is 2 sided ) so the pvalue is still 0.62.

So, it's probably doing the first case andf them multiplying it by 2 which is incorrect. Also, no offense intended but it's dangerous to use these things unless the understanding is there. In fact, it can be dangerous to use them even when the understanding is there !!!!!!! Peter Daalgard's book or John Verzani's book are probably decent recommendations to read for the above kind of thing but an introduction to statistical testing textbook is probably most useful. I can't think of a title at the moment.









On Sat, Mar 21, 2009 at  9:07 PM, C.H. wrote:

I tried the OpenEpi, the p-value of 1.25 is due to the fact that the
one tailed p-value is 0.62. The two tailed p-value then is 0.62 * 2 =
1.25. OpenEpi is not clever enough to ceiling the p-value to 1.

CH

On Sun, Mar 22, 2009 at 3:43 AM, David Winsemius <dwinsem...@comcast.net> wrote:
Let me ask you: What degree of credibility should be accorded a WWW
application that delivers a p-value of 1.25?

If the answer is not immediately and glaringly obvious, then tell us, what
sort of axioms of probability are you working with?

--
David Winsemius
On Mar 21, 2009, at 3:17 PM, Viju Moses wrote:

Hi, I noted a discrepancy between R and openepi when I ran a fisher test
with the same matrix. In R:

a=matrix(c(1,2,6,17), nrow=2)
a
  [,1] [,2]
[1,]    1    6
[2,]    2   17
fisher.test(a, conf.int=T)

     Fisher's Exact Test for Count Data

data:  a
p-value = 1
alternative hypothesis: true odds ratio is not equal to 1
95 percent confidence interval:
0.02061498 31.73691924
sample estimates:
odds ratio
1.396646

But in openepi the P value is 1.25. (In another instance too for other sets of data, I had got a p value of 1 in 3 instances for a prop.test when I got 3 other answers on a friend's stata software with the same data. )

I'm using R on Ubuntu Intrepid. Is there anything I'm doing wrong? Any
other packages I have to install?

Thanks in advance

Viju Moses

______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT

______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.




--
CH Chan
Research Assistant - KWH
http://www.macgrass.com

______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

Reply via email to