18.1 Multinomial distribution
18.1.1 Probability mass function
If \(K \in \mathbb{N}\), \(N \in \mathbb{N}\), and \(\theta \in \text{$K$-simplex}\), then for \(y \in \mathbb{N}^K\) such that \(\sum_{k=1}^K y_k = N\), \[ \text{Multinomial}(y|\theta) = \binom{N}{y_1,\ldots,y_K} \prod_{k=1}^K \theta_k^{y_k}, \] where the multinomial coefficient is defined by \[ \binom{N}{y_1,\ldots,y_k} = \frac{N!}{\prod_{k=1}^K y_k!}. \]
18.1.2 Sampling statement
y ~ multinomial(theta)
Increment target log probability density with multinomial_lupmf(y | theta).
Available since 2.0
18.1.3 Stan functions
real multinomial_lpmf(array[] int y | vector theta)
The log multinomial probability mass function with outcome array y
of size \(K\) given the \(K\)-simplex distribution parameter theta and
(implicit) total count N = sum(y)
Available since 2.12
real multinomial_lupmf(array[] int y | vector theta)
The log multinomial probability mass function with outcome array y
of size \(K\) given the \(K\)-simplex distribution parameter theta and
(implicit) total count N = sum(y) dropping constant additive terms
Available since 2.25
array[] int multinomial_rng(vector theta, int N)
Generate a multinomial variate with simplex distribution parameter
theta and total count \(N\); may only be used in transformed data and
generated quantities blocks
Available since 2.8