Definition X∼NB(r,p) Distribution of number of failures (x) needed to get the r-th success pmf: (r−1x+r−1)pr(1−p)x, x∈0,1,2,… mean: pr(1−p) var: p2r(1−p) mgf: pr[1−(1−p)et]−r, t<−ln(1−p)