American Occupational Therapy Association Evidence-Based Literature Review Project Levels of Evidence Rating System
| Level of Evidence | Definitions |
| Level I | Systematic reviews, meta-analyses, randomized controlled trials |
| Level II | Two groups, nonrandomized studies (e.g., cohort, case-control) |
| Level III | One group, nonrandomized (e.g., before and after, pretest and posttest) |
| Level IV | Descriptive studies that include analysis of outcomes (e.g., single-subject design, case series) |
| Level V | Case reports and expert opinion that include narrative literature reviews and consensus statements |
| Level of Evidence | Definitions |
| Level I | Systematic reviews, meta-analyses, randomized controlled trials |
| Level II | Two groups, nonrandomized studies (e.g., cohort, case-control) |
| Level III | One group, nonrandomized (e.g., before and after, pretest and posttest) |
| Level IV | Descriptive studies that include analysis of outcomes (e.g., single-subject design, case series) |
| Level V | Case reports and expert opinion that include narrative literature reviews and consensus statements |
Note. From “Evidence-Based Medicine: What It Is and What It Isn’t” by D. L. Sackett, W. M. Rosenberg, J. A. Muir Gray, R. B. Haynes, & W. S. Richardson, 1996, British Medical Journal, 312, pp. 71–72. Copyright © 1996 by the British Medical Association. Adapted with permission.