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24.4 Posterior predictive simulation in Stan

Posterior predictive quantities can be coded in Stan using the generated quantities block.

24.4.1 Simple Poisson model

For example, consider a simple Poisson model for count data with a rate parameter \(\lambda > 0\) following a gamma-distributed prior, \[ \lambda \sim \textrm{gamma}(1, 1). \] The likelihood for \(N\) observations \(y_1, \ldots, y_N\) is modeled as Poisson, \[ y_n \sim \textrm{poisson}(\lambda). \]

24.4.2 Stan code

The following Stan program defines a variable for \(\tilde{y}\) by random number generation in the generated quantities block.

data {
  int<lower = 0> N;
  int<lower = 0> y[N];
}
parameters {
  real<lower = 0> lambda;
}
model {
  lambda ~ gamma(1, 1);
  y ~ poisson(lambda);
}
generated quantities {
  int<lower = 0> y_tilde = poisson_rng(lambda);
}

The random draw from the sampling distribution for \(\tilde{y}\) is coded using Stan’s Poisson random number generator in the generated quantities block. This accounts for the sampling component of the uncertainty; Stan’s posterior sampler will account for the estimation uncertainty, generating a new \(\tilde{y}^{(m)} \sim p(y \mid \lambda^{(m)})\) for each posterior draw \(\lambda^{(m)} \sim p(\theta \mid y).\)

The posterior draws \(\tilde{y}^{(m)}\) may be used to estimate the expected value of \(\tilde{y}\) or any of its quantiles or posterior intervals, as well as event probabilities involving \(\tilde{y}\). In general, \(\mathbb{E}[f(\tilde{y}, \theta) \mid y]\) may be evaluated as \[ \mathbb{E}[f(\tilde{y}, \theta) \mid y] \approx \frac{1}{M} \sum_{m=1}^M f(\tilde{y}^{(m)}, \theta^{(m)}), \] which is just the posterior mean of \(f(\tilde{y}, \theta).\) This quantity is computed by Stan if the value of \(f(\tilde{y}, \theta)\) is assigned to a variable in the generated quantities block. That is, if we have

generated quantities {
  real f_val = f(y_tilde, theta);
  ...

where the value of \(f(\tilde{y}, \theta)\) is assigned to variable f_val, then the posterior mean of f_val will be the expectation \(\mathbb{E}[f(\tilde{y}, \theta) \mid y]\).

24.4.3 Analytic posterior and posterior predictive

The gamma distribution is the conjugate prior distribution for the Poisson distribution, so the posterior density \(p(\lambda \mid y)\) will also follow a gamma distribution.

Because the posterior follows a gamma distribution and the sampling distribution is Poisson, the posterior predictive \(p(\tilde{y} \mid y)\) will follow a negative binomial distribution, because the negative binomial is defined as a compound gamma-Poisson. That is, \(y \sim \textrm{negative-binomial}(\alpha, \beta)\) if \(\lambda \sim \textrm{gamma}(\alpha, \beta)\) and \(y \sim \textrm{poisson}(\lambda).\) Rather than marginalizing out the rate parameter \(\lambda\) analytically as can be done to define the negative binomial probability mass function, the rate \(\lambda^{(m)} \sim p(\lambda \mid y)\) is sampled from the posterior and then used to generate a draw of \(\tilde{y}^{(m)} \sim p(y \mid \lambda^{(m)}).\)