patients within a hospital). This two-tiered sampling framework allows for the estimation of random components at both the level of the group (Level 2: contributing study) and of the individual (Level 1: individual observation within study). However, a key practical requirement of this approach is that a sufficient number of groups (in our case, contributing studies) have been sampled to both obtain a representative sample of the population and to support the numerical estimation of the group-level random components. There are no firm rules on the required minimum number of groups, but in many situations at least 10 or 20 groups must be observed in order to allow for population representation and proper model estimation.
