Table 2.

Quality of evidence according to assessment system of the US Preventive Services Task Force, from Grimes and Schulz [33]

I	Evidence from at least one properly designed randomized control trial
II-1	Evidence obtained from well-designed controlled trials without randomization
II-2	Evidence from well-designed cohort or case-control studies, preferably from more than one centre or research group
II-3	Evidence from multiple time series with and without the intervention. Important results in uncontrolled experiments (such as introduction of penicillin treatment in the 1940s) could also be considered as this type of evidence
III	Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees