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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