# Beta negative binomial distribution

In probability theory, a beta negative binomial distribution is the probability distribution of a discrete random variable X equal to the number of failures needed to get r successes in a sequence of independent Bernoulli trials where the probability p of success on each trial is constant within any given experiment but is itself a random variable following a beta distribution, varying between different experiments. Thus the distribution is a compound probability distribution.

Parameters $\alpha >0$ shape (real)$\beta >0$ shape (real) $r>0$ — number of failures until the experiment is stopped (integer but can be extended to real) k ∈ { 0, 1, 2, 3, ... } ${\frac {\Gamma (r+k)}{k!\;\Gamma (r)}}{\frac {\mathrm {B} (\alpha +r,\beta +k)}{\mathrm {B} (\alpha ,\beta )}}$ ${\begin{cases}{\frac {r\beta }{\alpha -1}}&{\text{if}}\ \alpha >1\\\infty &{\text{otherwise}}\ \end{cases}}$ ${\begin{cases}{\frac {r(\alpha +r-1)\beta (\alpha +\beta -1)}{(\alpha -2){(\alpha -1)}^{2}}}&{\text{if}}\ \alpha >2\\\infty &{\text{otherwise}}\ \end{cases}}$ ${\begin{cases}{\frac {(\alpha +2r-1)(\alpha +2\beta -1)}{(\alpha -3){\sqrt {\frac {r(\alpha +r-1)\beta (\alpha +\beta -1)}{\alpha -2}}}}}&{\text{if}}\ \alpha >3\\\infty &{\text{otherwise}}\ \end{cases}}$ undefined ${\frac {\mathrm {B} (\alpha ,\beta +r)}{\mathrm {B} (\alpha ,\beta )}}{}_{2}F_{1}(r,\alpha ;\alpha +\beta +r;e^{it})\!$ where B is the beta function and 2F1 is the hypergeometric function.

This distribution has also been called both the inverse Markov-Pólya distribution and the generalized Waring distribution. A shifted form of the distribution has been called the beta-Pascal distribution.

If parameters of the beta distribution are α and β, and if

$X\mid p\sim \mathrm {NB} (r,p),$ where

$p\sim {\textrm {B}}(\alpha ,\beta ),$ then the marginal distribution of X is a beta negative binomial distribution:

$X\sim \mathrm {BNB} (r,\alpha ,\beta ).$ In the above, NB(r, p) is the negative binomial distribution and B(α, β) is the beta distribution.

## Definition

If $r$ is an integer, then the PMF can be written in terms of the beta function,:

$f(k|\alpha ,\beta ,r)={\binom {r+k-1}{k}}{\frac {\mathrm {B} (\alpha +r,\beta +k)}{\mathrm {B} (\alpha ,\beta )}}$ .

More generally the PMF can be written

$f(k|\alpha ,\beta ,r)={\frac {\Gamma (r+k)}{k!\;\Gamma (r)}}{\frac {\mathrm {B} (\alpha +r,\beta +k)}{\mathrm {B} (\alpha ,\beta )}}$ .

### PMF expressed with Gamma

Using the properties of the Beta function, the PMF with integer $r$ can be rewritten as:

$f(k|\alpha ,\beta ,r)={\binom {r+k-1}{k}}{\frac {\Gamma (\alpha +r)\Gamma (\beta +k)\Gamma (\alpha +\beta )}{\Gamma (\alpha +r+\beta +k)\Gamma (\alpha )\Gamma (\beta )}}$ .

More generally, the PMF can be written as

$f(k|\alpha ,\beta ,r)={\frac {\Gamma (r+k)}{k!\;\Gamma (r)}}{\frac {\Gamma (\alpha +r)\Gamma (\beta +k)\Gamma (\alpha +\beta )}{\Gamma (\alpha +r+\beta +k)\Gamma (\alpha )\Gamma (\beta )}}$ .

### PMF expressed with the rising Pochammer symbol

The PMF is often also presented in terms of the Pochammer symbol for integer $r$ $f(k|\alpha ,\beta ,r)={\frac {r^{(k)}\alpha ^{(r)}\beta ^{(k)}}{k!(\alpha +\beta )^{(r)}(r+\alpha +\beta )^{(k)}}}$ ## Properties

The beta negative binomial distribution contains the beta geometric distribution as a special case when $r=1$ . It can therefore approximate the geometric distribution arbitrarily well. It also approximates the negative binomial distribution arbitrary well for large $\alpha$ and $\beta$ . It can therefore approximate the Poisson distribution arbitrarily well for large $\alpha$ , $\beta$ and $r$ .

By Stirling's approximation to the beta function, it can be easily shown that

$f(k|\alpha ,\beta ,r)\sim {\frac {\Gamma (\alpha +r)}{\Gamma (r)\mathrm {B} (\alpha ,\beta )}}{\frac {k^{r-1}}{(\beta +k)^{r+\alpha }}}$ which implies that the beta negative binomial distribution is heavy tailed.