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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.