Fig. 3

In a, the hospital performs PTR less and has a higher threshold for PTR while in b, the hospital performs PTR more frequently and has a lower threshold for carrying out PTR. In both hospitals, there will be a population of patients who would never be performed PTR (group “a”) as well as patients who would always be performed PTR (group “c”). At the same time, there will be a group of patients who would either not be performed (group “b1”) or would be performed (group “b2”) PTR solely because of the hospital to which they presented. These “b” groups together are the marginal population