Table 1.
Typology and grade of evidence
| Type of study | Grade | Description |
|---|---|---|
| Type I | Good, Fair, Poor | Evidence obtained from at least one properly randomized controlled trial |
| Type II-1 | Good, Fair, Poor | Evidence obtained from well-designed controlled trials without randomization |
| Type II-2 | Good, Fair, Poor | Evidence obtained from well-designed cohort or case-control analytic studies, preferably from more than one centre or research group |
| Type II-3 | Good, Fair, Poor | Evidence obtained from multiple time series with or without the intervention (dramatic results in uncontrolled experiments could also be regarded as this type of evidence) |
| Type III | Good, Fair, Poor | Opinions of respected authorities, based on clinical experience, descriptive studies and case reports or reports of expert committees |
a
This hierarchy was copied verbatim (with one parenthetical removed) from Harris et al. (2001).