Actually, their methodology seems pretty sound, with robust findings, including in the subgroup of patients cared by hospitalists, where -- as the authors correctly point out -- patients are pretty much randomized to a female or male hospitalist.

The question is "in a given medical speciality -- internal medicine in this study --, are patient's outcomes related to the admitting physician sex?", not if women make better doctors than females. As such, your point about the best doctors not being in internal medicine is pretty a non sequitur.

Basically, I see two potential biais that could explain at least part of the difference in outcomes :

1. Its is "common knowledge" that male doctors tend to refuse admissions more often than females. (I am not aware of any hard data to prove this point) It could be that the patients that they do admit are somewhat sicker that those they let go directly home. That could explain both higher mortality and re-admission rates.

2. It is again, "common knowledge" (again, I'm not aware of any study) that women write better discharge summaries, including all co-morbid conditions. When coded in a database, those patients will appear sicker than similar ones treated by male doctors, and this could skew any analyses.

A final confounder is that patients often have many doctors during the stay, both male and female, and the study assigned as "the" doctor the one who billed most for the patient, which may not be the one who had the most impact on outcomes. It is difficult to say how this would advantage any particular sex, however.

I can find other potential bias that would explain the difference in outcomes, like for example internal competition. If female and male doctors move different within the profession and compete on different aspects (a common finding in many other professions), the outcome will also reflect that. The result might thus simply be explained that male doctors compete more for medical positions in areas outside of elder care.

If that data is true, then we should see a reversed result in other medical areas. It could be that if you wanted the best result of surgery, pick a male doctor. If you want the best result afterward, pick a female doctor.

Good point about not correcting for medical speciality. I'm neither a doctor nor a statistician, but surely that pretty much invalidates the whole thing?

They report results for "hospitalists", which is its own specialty. It's not a super-specialized field, in the way that some doctors are pediatric neuro-radio-oncologists, but the hospitalists have largely had similar training.