The data available to patients includes a doctor's gender, education, certification, and malpractice claim history. The Archives of Internal Medicine study found that doctors who are female, board-certified, and trained in the U.S. provide slightly better care than other doctors, with board certification carrying the most weight. These distinctions, however, were not statistically very large.
Setting aside physicians who should not be practicing medicine because they have abandoned, neglected, or abused their patients, though, I wonder how exactly to define "quality of care" among physicians. In the doctor/patient relationship, different patients value different qualities: some might prefer a doctor who is the same gender as them, or who has a certain bedside manner, or who received training from an institution they admire, or whose office staff are easy to work with.
A patient who has a strained relationship with their doctor might avoid calling or visiting their doctor or revealing some medically-relevant personal information, even when they were ill or needed follow-up care. Even if the doctor provided excellent care, this patient would probably be better off with a less competent doctor who worked with the patient better.
I'm all in favor of the practice of evidence-based medicine, in which treatment decisions are made based on the best available science. But to some extent, "quality of care" is about the quality of the relationship between the patient and the doctor, which is why it is difficult to measure.
In the end, I like The Lancet's take on this topic in its August 28 issue: that the answer to the question "what makes a good doctor?" depends on who is asking it:
The attributes of a good doctor vary according to the population surveyed. Patients value communication and care, colleagues seek competence and camaraderie, medical students prize cheerfulness. By contrast, admission panels focus on chemistry grades, as if knowledge of ionic bonds is somehow a proxy for the complex human and organisational bonds between doctors, their patients, and colleagues.