[R] t-test problem

[Joshua Wiley]

Look what happens when you use lower.tail=FALSE (see ?pt ).  If that
is not your problem, perhaps you can provide a short sample vector
that replicates your results?

Josh

On Wed, Jun 16, 2010 at 10:02 PM, Worik R <worikr at gmail.com> wrote:
> More:
>
> When the t-stat is > 0 should I use 'pt' differently?
>
> I have been checking my results and (except for the example I posted) all
> the inconsistencies occur when t>0
>
> Worik
>
> On Thu, Jun 17, 2010 at 10:30 AM, 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
>>
>
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>
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-- 
Joshua Wiley
Ph.D. Student
Health Psychology
University of California, Los Angeles