The paper's authors tried to take into account other factors, but they admit it may not have been entirely possible to quantify differences in, for example,the recipient's health, which can be a big factor in the success of a transplant.

VOA: standard.2010.07.14

The same problem is with organ transplantation is that the recipient has to be matched to the donor and so that's a problem.

和器官移植中的问题一样,受体需要与供体匹配,这就是问题所在

耶鲁公开课 - 生物医学工程探索课程节选