I am searching for the formula that will generate the critical values in a t-test instead of using a static critical value table.

I am programming in Java and not R so the qt() function is not what I am looking for.

Any help is appreciated.

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#### Best Answer

**The Student t distribution is essentially an inverse beta function,** which is defined in terms of an integral. A good resource for basic formulas for the common functions in statistics is the *Numerical Recipes* site (nr.com). The older editions, which include code in Fortran and C, are online. For the CDF of the Student t, see section 6.4 of the C version (which should port easily to Java and other C-like languages), p. 228; the inverse beta code is given on the immediately preceding pages.

**Some words of experience:** porting (or writing) numerical code is fraught with subtle perils and can introduce the nastiest kinds of bugs: those that give only slightly wrong answers and so go undetected. It is *essential* to create an exhaustive test suite. Consider tabulating a large array of values to high precision using reliable software like `R`

or *Mathematica* (*not* a spreadsheet!) and setting up a direct comparison of those values to yours–*before* you even begin coding. Really stretch the code by evaluating it at endpoints: supplying enormous values for the degrees of freedom, supplying fractional degrees of freedom (you will need this capability for other computations, anyway, such as in a Satterthwaite t-test), infinitesimally small positive values for the limit of integration, and so on. This is especially needed for t distributions because they are often the foundations of other calculations, such as noncentral t distributions, etc., and the algorithms in those calculations might supply surprising values of the arguments in their numerical searches. Robust, defensive coding is essential.

**You might consider using a well-tested library instead.** 🙂

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