Third, the sensitivities of the diagnosis
codes that we used to determine risk factors
for thrombosis have not been validated, and
therefore our subgroup analyses may not have
detected true interactions between specific risk
factors — especially between the presence of a
primary hypercoagulable state and smoking —
and the duration of thrombotic risk after delivery.
Fourth, we lacked data from federal health
care facilities, which comprise 3.1% of the facilities
in California