I am using proc logistic to investigate the association between the variables laek and pv (indexar, alder, arv, and koen are confounders). The model looks something like this:   proc logistic data=dataset; class indexar (ref='2010') koen (ref='K') / param = ref ; model laek(EVENT='1') = pv indexar alder  arv koen; run;   The individual level dataset that I am using is obtained from patients who are grouped within different clinics. I believe that the clinic effect is a mediator rather than a confunder. How can I adjust for clustering? Should I use a "sandwich" estimator and in that case, how do I do that in SAS? ... View more