Do high volume providers get better results because they perform procedures more often? Or do they perform procedures more often because of referrals from doctors who know they are good at what they do? This is Austin Frakt, writing at his blog (entire post is worth reading):
Hospitals that perform a greater volume of a specific procedure do it better, right? Well, actually, that’s believable and supported by evidence. Now for the harder question: it’s the greater volume that causes the better outcomes, right? You know, “learning by doing,” “practice makes perfect,” etc. …
In fact, there’s good reason to believe causality runs the other way too. Hospitals that yield better outcomes have higher volume, a referral effect. Ask 100 physicians in your area where to have a CABG and the results won’t be random. They’ll point you to the well-known facility or two that do the best job, perhaps with the lowest mortality. So, more patients will go to those, increasing their volume. Low mortality causes higher volume. That’s not “practice makes perfect.” That’s a referral effect.
A recent paper in Health Economics…[finds that] after controlling for the simultaneity of volume and mortality…“specialty hospitals do not have an advantage over general hospitals in mortality rates after cardiac revascularization.” … Therefore, efforts to increase volume may not themselves increase the quality of outcomes.