Rating	Description
United States Preventive Services Task Force1
I	Evidence obtained from at least one properly randomized, controlled trial.
II-1	Evidence obtained from well-designed controlled trials without randomization.
II-2	Evidence obtained from well-designed cohort or case–control analytic studies, preferably from more than one center or research group.
II-3	Evidence obtained from multiple time series with or without the intervention. Dramatic results in uncontrolled experiments (such as the results of the introduction of penicillin treatment in the 1940s) also could be regarded as this type of evidence.
III	Opinions of respected authorities based on clinical experience, descriptive studies and case reports, or reports of expert committees.
American College of Obstetricians and Gynecologists2
I	A randomized controlled trial.
II	A cohort or case–control study that includes a comparison group.
III	An uncontrolled descriptive study including case series.