Definition
r: required number of successes
p: probability of success
X ~ NB(r, p)
Conditions for a Negative Binomial Distribution
 The trials are independent of each other
 There are two possible outcomes, success or failure
 The probability of success is constant

Formulae
P(X = x) = ^{x1}C_{r1}p^{r}(1  p)^{xr}
= 
(x  1) 
(r  1)! (x  r)! 


