U.S. Preventive Service Task Force Criteria28 (modified by author)
Ia Properly randomized controlled trials
Ib Randomized, but with departures from best methods (sequential, date of birth, etc.); conduct otherwise acceptable
Ic As Ib, but conduct not adequate (high dropout rate (30+%), lack of blinding, etc.)
II-1 Well-designed controlled trials without randomization
II-2 Well-designed cohort or case-control analytic studies, preferably from more than one center or research group; multiple time series with or without intervention
II-2b Less rigorously conducted observational studies; high dropout rates, etc.
II-3 Dramatic results in uncontrolled experiments; opinions of respected authorities, based on clinical experience
III Descriptive studies and case reports, or reports of expert committees
NHLBI criteria for overall recommendations29
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