ogram is not cheating by using the test data during training. Although this model does not do so, it would be fair to use <code>x_tilde</code> in the model block—only the test observations <code>y_tilde</code> are unknown before they are predicted.</p>
<p>Given <span class="math inline">\(M\)</span> posterior draws from Stan, the sequence <code>log_p[1:M]</code> will be available, so that the log posterior predictive density of the test data given training data and predictors is just <code>log_sum_exp(log