[R] t-test pvalue

[Charles Annis, P.E.]

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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