實證醫學的三個依據原則

Systems to stratify evidence by quality have been developed, such as this 
one by the U.S. Preventive Services Task Force(USPSTF) for ranking
evidence about the effectiveness of treatments or screening:

 Level I: Evidence obtained from at least one properly designed randomized
 controlled trial.

 Level II-1: Evidence obtained from well-designed controlled trials without
 randomization.
 Level II-2: Evidence obtained from well-designed cohort or case-control
 analytic studies, preferably from more than one center or research group.
 Level II-3: Evidence obtained from multiple time series with or without the
 intervention. Dramatic results in uncontrolled trials might also be regarded
 as this type of evidence.

 Level III: Opinions of respected authorities, based on clinical experience,
 descriptive studies, or reports of expert committees.