scipy.special.nbdtr

scipy.special.nbdtr(x1, x2, x3[, out])

y=nbdtr(k,n,p) returns the sum of the terms 0 through k of the negative binomial distribution: sum((n+j-1)Cj p**n (1-p)**j,j=0..k). In a sequence of Bernoulli trials this is the probability that k or fewer failures precede the nth success.