[R] t-test problem

[Oscar Rueda]

Hi Worik, 

You can try
2*pt(abs(t1$statistic), t1$parameter, lower.tail=FALSE)

If the test is two sided.

Cheers, 
Oscar

Oscar M. Rueda, PhD
Postdoc, Breast Cancer Functional Genomics
Cancer Research UK Cambridge Research Institute
Li Ka Shing Centre 
Robinson Way 
Cambridge CB2 0RE 
England 

On 16/6/10 23:30, "Worik R" <worikr at gmail.com> wrote:

> I have two pairs of related vectors
> x1,y1
> 
> and
> 
> x2,y2
> 
> I wish to do a test for differences in means of x1 and y1, ditto x2 and y2.
> 
> I am getting odd results.  I am not sure I am using 'pt' properly...
> 
> I have not included the raw vectors as they are long.  I am interested if I
> am using R properly...
> 
>> c(length(x1), length(y1), length(x2), length(y2))
> [1] 3436 1619 2677 2378
> 
> 
> First where the T-stat and the DF do not give the same result as 't.test'
> when passed into 'pt'
> 
>> t.1 <- t.test(x1, y1)
>> 2 * pt(t.1$statistic, t.1$parameter)
>        t
> 1.353946
>> t.1$p.value
> [1] 0.646054
> 
> I would have thought these would have been the same.  Like below....
> 
>> t.2 <- t.test(x2, y2)
>> 2 * pt(t.2$statistic, t.2$parameter)
>         t
> 0.8679732
>> t.2$p.value
> [1] 0.8679732
> 
> This is what I expect.
> 
> clearly I misunderstand some thing.  What is it?
> 
> cheers
> Worik
> 
> [[alternative HTML version deleted]]
> 
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