From 【wiki】
最早1979年 “加拿大定期健康检查工作队”提出该标准【递减】
I-Evidence
Evidence from at least one randomized controlled trial
至少有一个随机对照试验获得的证据
II1-Evidence
Evidence from at least one well designed cohort study or case control study, i.e. a controlled trial which is not randomized
至少有一个合理设计的群组研究（cohort study）或者案例对照研究（case control study）, 即一个可控的非随机对照试验获得的证据
II 2-Evidence
Comparisons between times and places with or without the intervention
进行过干预或非干预的比较研究，比较不同的时间和地点（的案例）获得的证据
III-Evidence
Opinions of respected authorities, based on clinical experience, descriptive studies or reports of expert committees.
来自于权威机构的观点，基于临床经验、描述性研究或专家委员会的报告
加拿大定期健康检查工作队(CTF) 将他们的提议分为五分制，五档：
A: Good level of evidence for the recommendation to consider a condition,
基于扎实的证据而提议考虑某情况
B: Fair level of evidence for the recommendation to consider a condition,
基于合理的证据而提议考虑某情况
C: Poor level of evidence for the recommendation to consider a condition,
基于较弱的证据而提议考虑某情况
D: Fair level evidence for the recommendation to exclude the condition,
基于合理的证据而提议排除某情况
E: Good level of evidence for the recommendation to exclude condition from consideration.
基于扎实的证据而提议排除某情况
1988年美国预防服务工作队（United States Preventive Services Task Force(USPSTF)）基于CTF 提出如下标准：
Level I:
Evidence obtained from at least one properly designed randomized controlled trial.
至少从一个合理设计的随机对照试验获得的证据
Level II-1:
Evidence obtained from well-designed controlled trials without randomization.
从一个合理设计的非随机的对照试验获得的证据
Level II-2:
Evidence obtained from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.
从一个合理设计的群组研究（cohort study）或者案例对照研究（case control study）获得的证据, 最好来自多于一个研究中心或研究小组
Level II-3:
Evidence obtained from multiple time series designs with or without the intervention. Dramatic results in uncontrolled trials might also be regarded as this type of evidence.
从多个干预或非干预的时间序列获得的证据, 由于非控制而导致对照实验产生剧烈的结果差异，也归为此类
Level III:
Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees.
来自于权威机构的观点，基于临床经验、描述性研究或专家委员会的报告
Another example of a system for grading evidence is the Oxford (UK) CEBM Levels of Evidence, Most of the evidence ranking schemes grade evidence for therapy and prevention, but not for diagnostic tests, prognostic markers, or harm. The Oxford CEBM Levels of Evidence addresses this issue and provides 'Levels' of evidence for claims about prognosis, diagnosis, treatment benefits, treatment harms, and screening. The original CEBM Levels was first released in September 2000 for Evidence-Based On Call to make the process of finding evidence feasible and its results explicit. As published in 2009[9]they are :
1a:
Systematic reviews (with homogeneity) of randomized controlled trials
由随机对照实验得到的（同一主题的）综合观点
1b:
Individual randomized controlled trials (with narrow confidence interval)
单独的随机对照实验（置信区间小）
1c:
All or none randomized controlled trials
要么全部，要么没有任何一个，是随机对照实验【我不太理解为什么全部的随机对照实验是1c】
2a:
Systematic reviews (with homogeneity) of cohort studies
由群组研究得到的（同一主题的）综合观点
2b:
Individual cohort study or low quality randomized controlled trials (e.g. <80% follow-up)
单独的群组研究，或者质量较低的随机对照实验（比如小于80%后续实验）【对于这个例子我并不理解】
2c:
"Outcomes" Research; ecological studies
临床实效研究；生态学研究
3a:
Systematic review (with homogeneity) of case-control studies
由案例对比研究得到的（同一主题的）综合观点
3b:
Individual case-control study
单独的案例对比研究
4:
Case series (and poor quality cohort and case-control studies)
病历分析（也包括低质量的群组和案例对比研究）
5:
Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"
无明确的批判性鉴定的专家意见，或基于生理学的实验室研究或“第一原理（公理）”
In 2011, the Oxford CEBM Levels were redesigned by an international team to make it more understandable and to take into account recent developments in evidence ranking schemes. The Oxford CEBM Levels of Evidence have been used by patients, clinicians and also to develop clinical guidelines including recommendations for the optimal use of phototherapy and topical therapy in psoriasis[10] and guidelines for the use of the BCLC staging system for diagnosing and monitoring hepatocellular carcinoma in Canada.[11]
链接 en.wikipedia.org/wiki/Levels_of_evidence
en.wikipedia.org/wiki/Randomized_controlled_trial
随机对照试验（英语：randomized controlled trial，RCT）是一种对医疗卫生服务中的某种疗法或药物的效果进行检测的手段，特别常用于医学、药学、护理学研究中，在司法、教育、社会科学等其他领域也有所应用。
随机对照试验的基本方法是，将研究对象随机分组，对不同组实施不同的干预，在这种严格的条件下对照效果的不同。在研究对象数量足够的情况下，这种方法可以抵消已知和未知的混杂因素对各组的影响。
“临床实效研究”或“实效研究”（Outcomes Research），也有人称之为“效果研究”，是近年来在国际临床医学领域迅速发展起来的新学科。在许多临床试验（Clinical Trials）研究中，凡是有多种疾病、有多种并发症或服用多种药物的病人常常被排除在外。临床实效研究则是研究药物或其它医疗措施在非控制的真实临床情况下（real world）的效果及经济和社会效益等[1]。临床试验研究（如RCT）和实效研究对于充分了解一个医疗产品的实际医疗效果、效益和安全性有着同样重要的意义，只是方法学上有所不同。
第一原理（英语：First principle），哲学与逻辑名词，是一个最基本的命题或假设，不能被省略或删除，也不能被违反。第一原理相当于是在数学中的公理。最早由亚里斯多德提出。
