How can I get the P value of the two-tail Student's test when I know the T value and Degree of Freedom?

I have already known the T and Degree of Freedom. I try to calculate the P value with tpdf and tcdf. They output the different answers. I would like to konw which one is right. Thank you!

[the cyclist]

Can you share your parameters and the code that you used to calculate the P value? (You can use the paper clip icon in the INSERT section of the toolbar.)

I would expect you are going to need the cdf, not the pdf, to calculate the P value.

[Murong Qi]

This is an example of my code:

%eg:t-value:10 ; Degree of freedom: 20
%[method 1]:tpdf
p1 = tpdf(10,20);
%[method 2]:tcdf
p2 = 2*tcdf(-abs(10),20);

Answer 1

[the cyclist]

Sorry for the delayed response. I did not get a notification that you had replied.

Your second method is the correct one (assuming you want 2-tailed P value):

t = 10;
DOF = 20;
twoTailedPValue = 2*tcdf(-t,DOF)

twoTailedPValue = 3.1638e-09

I verified the P values at this site (using a smaller value of t, that site won't calculate such a small P value exactly).

I'm not sure why you think your first method would be correct. The pdf value is not directly relevant; you need the integral under the pdf (which is of course the cdf).

[Murong Qi]

I misunderstood the definition of P-value.Thank you so much for your answers. They help a lot.