ttest calls betai to determine the incomplete beta function for a Student's t test. The probability returned by betai is twice the probability in statistics textbook tables of the t distribution. Could someone please explain to me (a novice in statistics) why the betai function returns 2 x the t distribution?

Thanks.

I think that betai returns the 2-tailed (both sides of the curve) distribution, while your stats book returns a 1-tailed distribution.

Of course, I could be wrong :)