NASA Categories of Evidence comprise a matrix or scale denoting the sources of evidence provided in the Human Research Program's various evidence reports, and thus potentially their probative value and efficacy.[1] Authors in the Program were urged to label their evidence according to whether it was based on controlled experiments, observation, or expert opinion.
| Broad "Experimental" Design Type | Silagy & Haines Levels of Evidence[2] | Nasa Categories of Evidence |
|---|---|---|
| Controlled | Ia - Meta-analysis of randomized trials | I - At least one randomized, controlled trial |
| Ib - At least one randomized trial | ||
| IIa - At least one controlled study without randomization | II - At least one controlled study without randomization, including cohort, case-control, or subject operating as own control. | |
| IIb - At least one other quasi-experimental study | ||
| Observational | III - Non-experimental descriptive studies, e.g. comparative correlation or case studies. | III - Non-experimental observations or comparative, correlation, and case or case-series studies. |
| Opinion | IV - Expert committee reports or opinions or clinical experiences of respected authorities. | IV - Expert committee reports or opinions of respected authorities based on clinical experiences, bench research, or "first principles". |