Gray's Level of Evidence

Type Strength
I Strong evidence from at least one systematic review of multiple well-designed, randomized controlled trials.
II Strong evidence from at least one properly designed, randomized controlled trial of appropriate size.
III Evidence from well-designed trials without randomization, single group pre-post, cohort, time series, or matched case-control studies.
IV Evidence from well-designed, non-experimental studies from more than one center or research group.
V Opinions of respected authorities, based on clinical evidence, descriptive studies, or reports of expert committees.