Controlled hospital quality? I figure the best credentialed doctors go to the best hospitals, where patients receive a lot of other care aside from the MD.
>The researchers compared outcomes for patients within the same hospitals who were cared for by doctors with different exam scores. This allowed the researchers to eliminate, or at least minimize, the effect of differences in patient populations, hospital resources, and other variations that might influence the odds of patient death or readmission, independent of a doctor’s performance.
For example, maybe burn unit care is high-mortality and low-barrier, compared to sleep medicine which is low-mortality and high-barrier (I don't know how accurate this is, just providing some hypotheticals for clarity)