[R] Calculating p-value for 1-tailed test in a linear model

[Albyn Jones]

For H_0: beta >= 0, then the correct p-value is

         pt(tvalue,df)

regardless of the sign of tvalue.  Negative tvalues of large magnitude
will yield small p-values.

albyn

On Mon, Aug 22, 2011 at 05:22:06PM +0000, Ben Bolker wrote:
> Campomizzi, Andrew J <acampomizzi <at> neo.tamu.edu> writes:
> 
> > On 20/08/11 10:20, Andrew Campomizzi wrote:
> > > Hello,
> > >
> > > I'm having trouble figuring out how to calculate a p-value for a 1-tailed
> > > test of beta_1 in a linear model fit using command lm.  My model has only 1
> > > continuous, predictor variable.  I want to test the null hypothesis beta_1
> > > is>= 0.  I can calculate the p-value for a 2-tailed test using the code
> > > "2*pt(-abs(t-value), df=degrees.freedom)", where t-value and degrees.freedom
> > > are values provided in the summary of the lm.  The resulting p-value is the
> > > same as provided by the summary of the lm for beta_1.  I'm unsure how to
> > > change my calculation of the p-value for a 1-tailed test.
> > >
> 
>   Isn't it just 
> 
> pt(tvalue,df=degrees.freedom,lower.tail=FALSE)
> 
> if the value is positive (and expected to be positive) or
> 
> pt(tvalue,df=degrees.freedom)
> 
> if the value is negative (and expected to be negative)?
> 
>   In fact, if the value is in the expected direction, I think you
> can just leave out the multiplication by 2 and get the right answer ...
> 
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-- 
Albyn Jones
Reed College
jones at reed.edu