TABLE 2.

United States Preventive Services Task Force levels of evidence for grading clinical trials

LEVEL OF EVIDENCE	QUALITY OF EVIDENCE18
I	Evidence obtained from at least one properly designed, randomized, controlled trial
II-i	Evidence obtained from well designed controlled trials without randomization
II-ii	Evidence obtained from well designed cohort or case control analytical studies, preferably from more than one center or research group
II-iii	Evidence obtained from multiple time series with or without the intervention; dramatic results in uncontrolled experiments could also be regarded as this type of evidence
III	Opinions of respected authorities based on clinical experience, descriptive studies, or reports of expert committees
IV	Evidence inadequate because of problems of methodology (eg, sample size or length of comprehensiveness of follow-up or conflicts in evidence)
STRENGTH OF RECOMMENDATIONS
A	There is good evidence to support the use of the procedure
B	There is fair evidence to support the use of the procedure
C	There is poor evidence to support the use of the procedure
D	There is fair evidence to support the rejection of the use of the procedure
E	There is good evidence to support the rejection of the use of the procedure