I am trying to analyze correlated data with PHREG. The clustering / grouping variable is drug class. What is the best way of getting accurate estimates? OPTION A proc phreg data = data covsandwich(aggregate); class route (ref="0") drug_class / ref=first; model follow_up_time*discontinued(0) = drug_age route / rl=both; id drug_class; run; OPTION B proc phreg data = data covsandwich(aggregate); class route (ref="0") drug_class / ref=first; model follow_up_time*discontinued(0) = drug_age route /rl=both; strata drug_class; run; In other words what is the difference between strata and ID when trying to account for correlated data? Thank you very much.
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