| Strength of Evidence | Description |
|---|---|
| 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. |
| IIIa | Evidence 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). |
| IIIb | Evidence 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). |
| IV | Evidence from well-designed, nonexperimental 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. |
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