Negative Binomial 11/3

This is the number of trials until r successes is obtained in a sequence of Bernouill(p) trials.

 

$$P(X=k)=p \times \binom{k-1}{r-1} p^{r-1} q^{(k-r)}= \binom{k-1}{r-1} p^{r} q^{(k-r)}$$

In fact the Geometric is the special case where r=1.