> 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.
>
>
Should I choose lower.tail value based on the sign of the t-stat?

I note that when I use 2*pt(Tstat, Df, lower.tail=FALSE) I get inconsistent
results when Tstat<0.

If I say:
lt <- TRUE
if(Tstat > 0) lt <- FALSE
pt(Tstat, Df, lower.tail=lt) I get consistent results.

I am happy with the results but I would like to know why?

?pt gives no hints for why this happens.

cheers
Worik

> 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
> >
> >        [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
>
>
>
> --
> Joshua Wiley
> Ph.D. Student
> Health Psychology
> University of California, Los Angeles
>

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