Binomial and Poisson count models
Different count mechanisms require different distributions.
Key knowledge
A binomial variable counts successes in n independent Bernoulli trials with common probability p: P(X=k)=C(n,k)pᵏ(1−p)ⁿ⁻ᵏ. A Poisson law has P(Y=k)=e⁻λ λᵏ/k! for nonnegative integer k.
Conditions and scope
- For binomial, n is a nonnegative integer, 0≤p≤1 and 0≤k≤n; use the limiting convention at p=0 or 1.
- For Poisson, λ>0; arbitrary count data do not establish this model.
Sources · 2
References
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Evidence and review
Section 3.5, physical pages 57 and 62: binomial and Poisson mass formulas. Fixed independent Bernoulli trials, common probability and binomial mean/variance.
2026-10-10 · Count-mass formulas and binomial trial assumptions.
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