Table 8 Discrete probability distributions
Name Abbreviation Probability function p(x) Range ĢĢX Probability generating function ĢĢX(s) Mean ĢĢX Variance ĢĢX^2
Bernoulli B(1, p) p^x q^{1-x} 0, 1 q + ps p pq
Binomial B(n, p) (n choose x) p^x q^{n-x} 0, 1, ..., n (q + ps)^n np npq
Discrete uniform 1/n 1, 2, ..., n [s(1-s^n)]/[n(1-s)] (n+1)/2 (n^2-1)/12
Poisson Poisson(ĢĢ) [e^{-ĢĢ}ĢĢ^x]/x! 0, 1, ... e^{-ĢĢ(1-s)} ĢĢ ĢĢ
Geometric (1, 2, ...) G1(p) q^{x-1}p 1, 2, ... ps/(1-qs) 1/p q/p^2
Negative binomial (r, r+1, ...) (x-1 choose r-1) q^{x-r} p^r r, r+1, ... [ps/(1-qs)]^r r/p rq/p^2
Geometric (0, 1, ...) G0(p) p^x q 0, 1, ... q/(1-ps) p/q p/q^2
Negative binomial (0, 1, ...) (r+x-1 choose r-1) p^x q^r 0, 1, ... [q/(1-ps)]^r rp/q rp/q^2
Modified geometric { a/c for x=0, [(ad-bc)/c^2](d/c)^{x-1} for x=1, 2, ... } 0, 1, 2, ... (a-bs)/(c-ds) (ad-bc)/(c-d)^2 [2d(ad-bc)]/(c-d)^3 + ĢĢ - ĢĢ^2