Discrepancy with p.value from t.test

t.out <- t.test(b1,b2,var.equal=T)
t.out$p.value

t.out$statistic

Sometimes the p.value returned by t.test() is the same that I calculate using pt() and sometimes it's not. I don't understand the difference. I'm sure there is a simple explanation but I haven't been able to find it, even after looking at the code for t.test.default. I apologize if this is a basic and obvious question. For example:

data(sleep)
t.test(extra~group,data=sleep,var.equal=T)

# the p.value returned is 0.07939

2*pt(-1.8608,18)   # using the t.statistic and the df returned above

[1] 0.0791887

These p.values are the same. However, they are different when I use a different dataset:

data(beavers)
b1 <- beaver1$temp
b2 <- beaver2$temp
t.test(b1,b2,var.equal=T)

# the p.value returned is 2.2e-16

2*pt(-15.9366,212)   # using the t.statistic and the df returned above

[1] 4.10686e-38


Jonathan B. Thayn, Ph.D.
Illinois State University
Department of Geography and Geology
200A Felmley Hall
Normal, Illinois 61790

(309) 438-8112
jthayn at ilstu.edu
my.ilstu.edu/~jthayn




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