# Parametrization of the negative binomial and gamma distribution

#### Klaus K. Holst

The negative binomial distribution describes the probability of seeing a given number of failtures failures before obtaining \(r\) successes in iid Bernoulli trials with probability parameter \(p\). More generally, let \(X\sim \operatorname{NB}(r, p)\) for real parameters \(r>0\), \(p\in(0,1)\), then \(X\) has distribution given by \[\begin{align*} \mathbb{P}(X=x) = \frac{\Gamma(r+x)}{k!\Gamma(r)}(1-p)^x p^r, x\in\mathbb{N}_0. \end{align*}\] The `stats::rbinom` function uses this parametrization, and we have \[\begin{align*} \mathbb{E}X = r(1-p)p^{-1}, \quad \mathbb{V}\!\!\operatorname{ar}(X) = r(1-p)p^{-2}. \end{align*}\] Further, if \(X_1\sim\operatorname{NB}(r_{1}, p)\) and \(X_2\sim\operatorname{NB}(r_{2}, p)\) are independent then \(X_1+X_2\sim\operatorname{NB}(r_{1}+r_{2}, p)\).

To simulate from a negative binomial distribution with specific mean and variance we can use the `carts::rnb` function

    x <- carts::rnb(1e4, mean = 1, variance = 2)
    c(mean(x), var(x))

    ## [1] 0.999300 1.974097

The negative binomial distribution can also be constructed as a gamma-poisson mixture. We write \(Z\sim\Gamma(\alpha, \beta)\) when \(Z\) is gamma-distributed with shape \(\alpha>0\) and rate parameter \(\beta>0\), and the density function is given by \[\begin{align*} f(z) = \frac{\beta^\alpha}{\Gamma(\alpha)}z^{\alpha-1}\exp(-\beta z), \quad z>0. \end{align*}\]

This parametrization leads to \[\begin{align*} \mathbb{E}Z= \alpha\beta^{-1}, \quad \mathbb{V}\!\!\operatorname{ar}(Z) = \alpha\beta^{-2}. \end{align*}\]

We will exploit that the gamma distribution is closed under both convolution and scaling. Let \(Z_{1}\sim\Gamma(\alpha_{1}, \beta)\) and \(Z_{2}\sim\Gamma(\alpha_{2}, \beta)\) be independent and \(\lambda>0\) then \[\begin{align*} Z_1 + Z_2 \sim \Gamma(\alpha_1+\alpha_2, \beta), \quad \lambda Z_1 \sim \Gamma(\alpha_1, \lambda^{-1}\beta). \end{align*}\]

    b <- 2
    a1 <- 1
    a2 <- 3
    r <- 0.3
    n <- 1e4
    ## Shape-rate parametrization
    z1 <- rgamma(n, a1, b)
    z2 <- rgamma(n, a2, b)
    ## r(z1+z2) ~  gamma(a1+a2, b/r)
    plot(density(r * (z1 + z2)))
    curve(dgamma(x, a1 + a2, b / r), add = TRUE, col = "red", lty = 2)

The negative binomial distribution now appears as the gamma-poisson mixture in the following way. If we let \(X\mid\lambda\sim\operatorname{Pois}(\lambda)\) with stochastic rate \(\lambda\sim\Gamma(\alpha, \beta)\), then \(X\sim\operatorname{NB}(\alpha, \beta(1+\beta)^{-1})\).

Consider now \(Z\sim\Gamma(\nu, \nu)\), hence \(\mathbb{E}Z=1\) and \(\mathbb{V}\!\!\operatorname{ar}(Z)=\nu^{-1}\), and let \(Y\mid Z\sim\operatorname{Pois}(\lambda_{0}Z)\) for some fixed \(\lambda_0>0\), then direct calculations shows that \(Y\sim\operatorname{NB}(\nu, \nu(\nu + \lambda_{0})^{-1})\) and \[\begin{align*} \mathbb{E}Y = \lambda_0, \quad \mathbb{V}\!\!\operatorname{ar}(Y) = \lambda_0^2\nu^{-1} + \lambda_0. \end{align*}\]

    z <- carts::rnb(1e5,
      mean = 2,
      gamma.variance = 3
    )
    c(mean(z), var(z))

    ## [1]  1.98525 13.72705

    x <- seq(0, 30)
    mf <- function(y, x) sapply(x, function(x) mean(y == x))
    plot(x, mf(z, x), type = "h", ylab = "p(x)")
    y <- rpois(length(z), 2 * rgamma(length(z), 1 / 3, 1 / 3))
    lines(x + 0.1, mf(y, x), type = "h", col = "red")

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