i did not read the study. an obvious confounding factor is that doctors with better board scores are hired into better hospitals with better patient populations, and thus better outcomes.
> 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.
This is also pretty much the easiest thing the factor out through mixed effects modeling (among other methods if required). But your statement that higher scoring physicians go to places with healthier patient populations is not correct across all disciplines. Often it can be the opposite: the best physicians go to the major hospitals (usually but not always university affiliated) located in major population centers that draw in the sickest/worst/rarest cases from the surrounding geography.
> 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.