Consider X∼Negative Binomial(r,p)X \sim \text{Negative Binomial}(r, p)X∼Negative Binomial(r,p) (number of failures before rrr successes). What is the variance?
r(1−p)/p2r(1-p)/p^2r(1−p)/p2
rp/p2rp/p^2rp/p2
r(1−p)/pr(1-p)/pr(1−p)/p
r/p2r/p^2r/p2