This is documentation for an old release of SciPy (version 0.12.0). Read this page in the documentation of the latest stable release (version 1.15.1).
Compute the incomplete beta integral of the arguments, evaluated from zero to x:
gamma(a+b) / (gamma(a)*gamma(b)) * integral(t**(a-1) (1-t)**(b-1), t=0..x).
Notes
The incomplete beta is also sometimes defined without the terms in gamma, in which case the above definition is the so-called regularized incomplete beta. Under this definition, you can get the incomplete beta by multiplying the result of the scipy function by beta(a, b).