| Favoring association |
| Practice makes perfect: increased volumes can lead to more proficient surgeons |
| Better ability to rescue patients from complications |
| Better implementation of process measures known to improve outcomes (e.g., administration of antibiotics before an operation) |
| Better teams leads to better systems of care |
| Fewer errors |
| Better technology, ICUs, support staff |
| Fallacies in association |
| Confounding by unmeasured patient characteristics-only patients with better performance status will travel to high volume centers |
| Selective referral bias: better outcomes in a center attract more patients leading to higher volumes |
| Higher hospital volume does not accurately reflect higher surgeon volume, better process measure implementation, or better ability to rescue |
