Are some hospitals better at giving medicine than others?

Research has repeatedly shown that, as expected, the outcomes of surgery depend highly on the skill and experience of the surgeon and the hospital team. So when researchers were designing a large, multi-center study comparing the use of stents and medications in stroke patients, they established a rigorous credentialing process to ensure the results wouldn't be skewed by the skill levels of the doctors placing the stents at the various centers. But when researchers did a follow-up analysis to look at the differences between the hospitals that enrolled more or fewer patients, they found a surprising result. There was no difference in outcomes for patients who received stents between higher or lower volume centers.