# Easy approximation of the incomplete beta function $\text{B}_x(a,b)$

I need to calculate $\text{B}_x(a,b)$ on the cheap, without too many coefficients and loops. For the complete $\text{B}(a,b)$, I can use $\Gamma(a)\Gamma(b)/\Gamma(a+b)$, and Stirling's approximation for $\Gamma(.)$. Is there a way to calculate the incomplete beta function using Stirling's approximation?

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To give a genuinely useful answer, you need to specify the specific ranges of $a$, $b$, and $x$ you are interested in. Methods that, for instance, work nicely for "human-sized" $a$ and $b$ (e.g. Eric's suggestion) fail spectacularly when either of $a$ or $b$ is large. If $x$ is outside $[0,1]$, special methods are needed, too... – J. M. Jul 27 '11 at 2:11
FWIW: due to the properties of ${}_2 F_1$, incomplete beta satisfies three term recurrence relations, which might be helpful if you'll be fixing some parameters. If for instance your goal is to numerically compute the CDF for the $\mathbf F$ or Student $t$ distributions, there are specially adapted methods for that as well. – J. M. Jul 27 '11 at 2:14

You can express ${\rm B}(x;a,b)$ in terms of the hypergeometric function
$$F(a,b;c;z) = 1 + \frac{{ab}}{c}z + \frac{{a(a + 1)b(b + 1)}}{{c(c + 1)2!}}z^2 + \cdots ,$$ as $${\rm B}(x;a,b) = \frac{{x^a }}{a}F(a,1 - b;a + 1;x)$$ or $${\rm B}(x;a,b) = \frac{{x^a (1 - x)^b }}{a}F(a + b,1;a + 1;x).$$ For this and more, see here.
I believe that one of the fastest ways to compute $\text{B}(a,b,x)$ is to simply use numerical integration. The integrand is very nice, and the interval is small. In fact, I think given a fixed amount of time, numerical integration will yield higher accuracy for $\text{B}(a,b)$ then Stirlings formula.