 example, in a simple natural language application, data might be structured by document. For cross-validation, one needs to cross-validate at the document level, not at the individual word level. This is related to <a href="../stan-users-guide/posterior-predictive-checks.html#mixed-replication">mixed replication in posterior predictive checking</a>, where there is a choice to simulate new elements of existing groups or generate entirely new groups.</p>
<p>Education testing applications are typically groupe