While the primary objective of multicenter clinical trials is to compare treatments for a specific disease, they also contain a lot of relevant additional information. For example, it may be interesting to investigate heterogeneity in outcome and in treatment effect over centers. Considering a frailty model including a random center effect and a random treatment by center interaction term, we demonstrate how to quantify this heterogeneity, to interpret it based on medically relevant quantities, and to identify center and patient specific factors explaining it. Based on the same...