limitation的几种写法
原则：有一些致命性的limitation不要提
一般流程：存在什么问题？没有解决这个问题的原因？以后要怎么办（也可以不写）？
第一句通常会说This study is not without limitations./ Our study has limitations./Our study had several major limitations.
1.Sample size较小
2.伦理问题：Owing to an ethical reason, we cann't/can only do sth
3.Selection bias选择偏倚〇的问题: Relatively less disabled patients were
recruited for better compliance to treatment and imaging follow-up
This study was performed in a selected population with…. Which may limit the generalization of the findings.
4.是否是在一个selected population： 例如1.Our study excluded patients with prior
CABG, and thus the utility of FFR-CT in this population remains unknown.
Future studies will be required to determine its diagnostic performance
in this population.
5.其他的一些不足：根据每个study的特点来定