# [R] t-test pvalue

Spencer Graves spencer.graves at pdf.com
Thu Dec 30 17:58:03 CET 2004

```      This may belabor the obvious, but the following packages Mr.

pt2 <- function(q, df,
log.p=FALSE){
2*pt(-abs(q), df, log.p=log.p)
}

> pt2(2.23, 10)
[1] 0.04984247
> pt2(-2.23, 10)
[1] 0.04984247
> pt(-2.23, 10)
[1] 0.02492124
>
hope this helps.  spencer graves

Charles Annis, P.E. wrote:

>pt(q, ...) returns the area to the left of q.  The area to the left of 2.23
>for your situation is 0.975, while the area to the left of -2.23 (which is
>on the left side of zero from 2.23) is 0.025.
>
>
>
>
>
>Charles Annis, P.E.
>
>Charles.Annis at StatisticalEngineering.com
>phone: 561-352-9699
>eFax:  614-455-3265
>http://www.StatisticalEngineering.com
>
>-----Original Message-----
>From: r-help-bounces at stat.math.ethz.ch
>[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Giovanni Coppola
>Sent: Wednesday, December 29, 2004 8:41 PM
>To: r-help at stat.math.ethz.ch
>Subject: [R] t-test pvalue
>
>Hi all,
>I have some t-test values, and I am trying to obtain the associated
>p-values.
>Is 'pt' the right command? I wonder why 1) it returns different values for
>x and -x, and 2) how to obtain a 2-sided p-value.
>
>example [R version 2.0.1, WinXP]:
>#if t=2.23 (df=10), the expected p-value is 0.05 for 2-sided and 0.025 for
>1-sided t-test
>
>pt (2.23,10)
>[1] 0.9750788
>
>pt (-2.23,10) #or pt (2.23,10,lower.tail=FALSE)
>[1] 0.02492124 #as expected
>
>The opposite happens starting from negative t-test values.
>
>Should I convert in negative values my t-test values, and leave
>lower.tail=TRUE?
>Thanks and happy new year
>Giovanni
>
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