Table 1.

Levels of evidence according to the United States Preventive Services Task Force [21], [22].^a

Level	Origin of evidence
I	Evidence obtained from at least one properly designed 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 Marked results in uncontrolled experiments might also be regarded as this type of evidence
III	Opinions of respected authorities on the basis of clinical experience, descriptive studies, or reports of expert communities

^aReproduced from Harris et al. [21], by permission of Elsevier.