I am trying to estimate variance components from a complex random effects survival model using coxme (Package version 2.2-5). I am having problems specifying interaction effects in the random terms. I wonder if this is simply not implemented in coxme yet. For instance, using the model coxme(Surv(DeathAge,Dead) ~ treatment + (1 | block/tank)) Works fine (tank nested within block), but coxme(Surv(DeathAge,Dead) ~ treatment + (1 | block) + (1 | block:tank)) fails with "Error in getgroups(f2$group, m) : Invalid grouping factor block:tank?. By my understanding of model specification in R, these should be equivalent statements (assuming that that the tank effects have been coded appropriately as unique within blocks). Any interaction specified using ?:? appears to fail. I am actually interested in much more complex models but can?t seem to be able to specify any interactions effects. For instance a factorial model like, coxme(Surv(DeathAge,Dead) ~ treatment + (1 | room*block)) fails with "Error in getgroups(f2$group, m) : Invalid grouping factor room * block?. Since I can?t directly specify the interaction effects and the factorial operator ?*? does not seem to work, I appear to be SOL. It seems that only nested effects specified by the ?/? operator are allowed. Multiple nesting is also OK, which is great for a single hierarchical nesting design, but not sufficient for more complex random effects models. Any thoughts would be most welcome. Thanks, Patrick