multi-level models remember features of each cluster in the data as they learn about all of the clusters depending on the variation across clusters, the model pools information across clusters the pooling improves estimates about each cluster benefits of the multilevel approach: improved estimates for repeat sampling improved estimates for imbalance in sampling estimates of variation avoid averaging and retain variation multilevel regression should be the default approach this chapter starts with the foundations and the following two are more advanced types of multilevel models 12.