Equivalence to Conditional Gamma-Poisson Model
The Dirichlet-multinomial distribution with parameter \(\alpha\) is equivalent to a conditional Gamma-Poisson model. This equivalence provides an alternative interpretation and can be useful for understanding the properties of the distribution.
Consider a Gamma-Poisson hierarchy where:
- \(U_k \sim \text{Gamma}(\alpha, 1)\)
- \(\lambda_k = \mu_k U_k\)
- \(y_k \mid \lambda_k \sim \text{Poisson}(\lambda_k)\) for \(k = 1, \ldots, K\)
Given \(\lambda_k\), the distribution of \(y_k\) is Poisson:
\[\begin{equation} y_k \mid \lambda_k \sim \text{Poisson}(\lambda_k) \end{equation}\]
The distribution of \(\mathbf{y}\) conditioned on the sum given the \(\lambda_k\) is multinomial:
\[\begin{equation}\label{eq:multinom} P(\mathbf{y} \mid \boldsymbol{\lambda}, \sum_{k=1}^K Y_k = N) = \frac{N!}{\prod_{k=1}^K y_k!} \prod_{k=1}^K \left(\frac{\lambda_k}{\sum_{j=1}^K \lambda_j}\right)^{y_k} \end{equation}\]
Integrating out the \(U_k\) gives the Dirichlet-multinomial distribution:
\[\begin{equation}\label{eq:dm} P(\mathbf{y} \mid \alpha, \boldsymbol{\mu}, N) = \frac{N!}{\prod_{k=1}^K y_k!} \frac{\Gamma(\alpha)}{\Gamma(N+\alpha)} \prod_{k=1}^K \frac{\Gamma(y_k+\alpha \mu_k)}{\Gamma(\alpha \mu_k)}. \prod_{k=1}^K \frac{\Gamma(y_k+\alpha\mu_k)}{\Gamma(\alpha\mu_k)}. \end{equation}\]
Note that \(\text{E}(U_k) = 1\) and \[ \text{sd}(U_k) = 1/\sqrt(\alpha) \]
Simplificaiton
Here is the simplification of the Dirichlet-multinomial PMF using the gamma recurrence identity.
The key identity is the gamma recurrence over an integer number of steps:
\[\begin{equation}\label{eq:gamma_ratio} \frac{\Gamma(x+n)}{\Gamma(x)} = x(x+1)\cdots(x+n-1) \end{equation}\]
Equation \(\ref{eq:dm}\) becomes
\[\begin{equation}\label{eq:dm_ratio} P(\mathbf{y} \mid \alpha, \boldsymbol{\mu}, N) = \frac{N!}{\prod_{k=1}^K y_k!} \frac{1}{\prod_{j=0}^{N-1} (\alpha + j)} \prod_{k=1}^K \prod_{j=0}^{y_k-1} (\alpha \mu_k + j) \end{equation}\]
substitute \[ \alpha = \frac{1}{\tau^2} \] to give \[\begin{equation}\label{eq:dm_ratio_tau} P(\mathbf{y} \mid \tau, \boldsymbol{\mu}, N) = \frac{N!}{\prod_{k=1}^K y_k!} \frac{\tau^{2N}}{\prod_{j=0}^{N-1} (1 + j \tau^2)} \prod_{k=1}^K \frac{\prod_{j=0}^{y_k-1} (\mu_k + j \tau^2)}{\tau^{2y_k}} \end{equation}\]
The \(\tau^2\) cancel. On the log scale:
\[\begin{equation}\label{eq:dm_ratio_log} \log P(\mathbf{y} \mid \tau, \boldsymbol{\mu}, N)= \log(N!) - \sum_{k=1}^K \log(y_k!) - \sum_{j=0}^{N-1} \log(1 + j \tau^2) + \sum_{k=1}^K \sum_{j=0}^{y_k-1} \log(\mu_k + j \tau^2) \end{equation}\]
Note that when \(\tau = 0\) then
\[\begin{equation}\label{eq:dm_ratio_log_zero} \log P(\mathbf{y} \mid \tau=0, \boldsymbol{\mu}, N)= \log(N!) - \sum_{k=1}^K \log(y_k!) + \sum_{k=1}^K y_k \log(\mu_k) \end{equation}\]
which is the multinomial density
K <- 6
N <- 100 * K
mu <- 2
tau <- 0.2
mu_vec <- rep(mu, K)
alpha <- mu_vec / sum(mu_vec)
U <- rgamma(N, shape = tau^(-2), rate = tau^(-2))
c(mean(U), sd(U))## [1] 1.0084683 0.2015098lambda <- mu * U
y <- rpois(N, lambda)
cc_matrix <- Matrix::sparseMatrix(
i = 1:N, j = rep(seq(1, N / K), each = K), x = 1
)
mu_vec <- rep(mu, K)
alpha_norm <- mu_vec / sum(mu_vec)
alpha <- alpha_norm / tau^2
log_dens_with_gammas <- log_dens_simplified <- rep(NA, ncol(cc_matrix))
for (D in 1:ncol(cc_matrix)) {
# Extract the indices where the column has non-zero values
indices_here <- which(cc_matrix[, D] == 1)
y_here <- y[indices_here]
# Simplified log density using the sum formula
# Handle zeros correctly by filtering them out for the sum
y_double_sum <- mapply(seq,
len = y_here,
from = alpha_norm,
MoreArgs = list(by = tau^2)
)
log_dens_simplified[D] <-
lfactorial(sum(y_here)) -
sum(lfactorial(y_here)) -
sum(log(seq(from = 1, by = tau^2, len = sum(y_here)))) +
sum(log(unlist(y_double_sum)))
# Log density using Gamma functions
# lgamma is defined for all non-negative integers, so no special handling needed for zeros
log_dens_with_gammas[D] <-
lgamma(sum(alpha)) + lgamma(sum(y_here) + 1) -
lgamma(sum(y_here) + sum(alpha)) +
sum(lgamma(y_here + alpha)) -
sum(lgamma(alpha)) - sum(lgamma(y_here + 1))
}