# Thread: inverse of the student`s t-distribution

1. ## inverse of the student`s t-distribution

Hi All,

I am trying to use calculate for two-tailed inverse of the student`s t-distribution function presented by Excel functions like =TINV(probability, deg_freedom)

For instance: The Excel function below;
N= 1223
Alpha= 0.05 (for 95%)

Then;
= TINV(0.05/1223,1223-2)
= TINV(0000408831, 1221)
= 4.0891672

I already got the equation for above excel function but does not work for probability (alpha/N) and just work probability alpha 0.05. I can calculate inverse Student-t value using approximations values for X, Y and Z. The equation below:

-------------------------------------------------
Tinv = exp(X + Y / df + Z / df2)
-------------------------------------------------

Another calculator by webpage on http://www.danielsoper.com/statcalc3/calc.aspx?id=10 but I don`t know this equation.

The point is how to calculate that value by hand for TINV like excel or webpage?

Cheers! Andre

2. ## Re: inverse of the student`s t-distribution

Why do you want to do this by hand? There isn't really a nice closed form expression for the CDF or the inverse (unless you have some special functions already programmed). Why do you want to do this by hand?

3. ## Re: inverse of the student`s t-distribution

Hi Dason,

I would like to use for my thesis in geological research. I want know how to calculate by hand step by step. Do you have softcopy textbook?

I hope that someone can solve my problems.

Cheers!

4. ## Re: inverse of the student`s t-distribution

it's not really something you can do by hand. There are approximations but there is no closed form solution. It's not really an enlightening endeavor either - is there a reason you can't just use software?

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