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

[Ben Bolker]

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 ...