TABLE 1 Gray Scale of Strength of Evidence
Strength of EvidenceDescription
IStrong evidence from at least one systematic review of multiple well-designed, randomized controlled trials.
IIStrong evidence from at least one properly designed, randomized controlled trial of appropriate size.
IIIaEvidence from well-designed trials/studies without randomization that include a control group (e.g., quasi-experimental, matched case-control studies, pre-post with control group).
IIIbEvidence from well-designed trials/studies without randomization that do not include a control group (e.g., single group pre-post, cohort, time series/interrupted time series).
IVEvidence from well-designed, nonexperimental studies from more than one center or research group.
VOpinions of respected authorities, based on clinical evidence, descriptive studies, or reports of expert committees.
  • Note: Gray includes 5 levels of evidence. For the “What Works” compendiums, level III was subdivided to differentiate between studies and evaluations whose design included control groups (IIIa) and those that did not (IIIb).101 Qualitative studies can be classified as either level IV or V, depending on number of study participants and other factors. For more detail about these types of studies and their strengths and weaknesses, see Gray (2009).7