The gamma distribution represents continuous probability distributions of two-parameter family. Gamma distributions are devised with generally three kind of parameter combinations.
A shape parameter k and a scale parameter θ.
A shape parameter α=k and an inverse scale parameter β=1θ, called as rate parameter.
A shape parameter k and a mean parameter μ=kβ.
Each parameter is a positive real numbers. The gamma distribution is the maximum entropy probability distribution driven by following criteria.
E[X]=kθ=αβ>0 and is fixed.E[ln(X)]=ψ(k)+ln(θ)=ψ(α)−ln(β) and is fixed.
Where −
X = Random variable.
ψ = digamma function.
Probability density function of Gamma distribution is given as:
Where −
α = location parameter.
β = scale parameter.
x = random variable.
Cumulative distribution function of Gamma distribution is given as:
F(x;α,β)=∫x0f(u;α,β)du=γ(α,βx)Γ(α)
Where −
α = location parameter.
β = scale parameter.
x = random variable.
γ(α,βx) = lower incomplete gamma function.
Probability density function of Gamma distribution is given as:
Where −
k = shape parameter.
θ = scale parameter.
x = random variable.
Γ(k) = gamma function evaluated at k.
Cumulative distribution function of Gamma distribution is given as:
F(x;k,θ)=∫x0f(u;k,θ)du=γ(k,xθ)Γ(k)
Where −
k = shape parameter.
θ = scale parameter.
x = random variable.
γ(k,xθ) = lower incomplete gamma function.