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Calcite | Level 5

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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