[R] t-test problem

[Joshua Wiley]

On Wed, Jun 16, 2010 at 3:30 PM, 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

Sorry, I realized that is is fairly easy to test that it is an issue
with which tail of the distribution you use.  This should show what is
going on better than my prior message.

1.353946/2 = 0.676973
1 - 0.676973 = 0.323027
0.323027 * 2 = 0.646054

in pt(), the default is lower.tail=TRUE.  Switching it to FALSE just
looks at the other tail of the distribution.  It was difficult to see
because of the multiplication by 2.

Josh


>> 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
>
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>
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-- 
Joshua Wiley
Ph.D. Student
Health Psychology
University of California, Los Angeles