# [R] t-test pvalue

Charles Annis, P.E. Charles.Annis at StatisticalEngineering.com
Thu Dec 30 02:53:08 CET 2004

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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