title: NASA categories of evidence
text: # NASA categories of evidence
  
  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. 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 | Nasa Categories of Evidence |
  | - | - | - |
  | Controlled | Ia - Meta-analysis of randomized trials | I - At least one randomized, controlled trial |
  | Controlled | Ib - At least one randomized trial | I - At least one randomized, controlled trial |
  | Controlled | 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. |
  | Controlled | IIb - At least one other quasi-experimental study | II - At least one controlled study without randomization, including cohort, case-control, or subject operating as own control. |
  | 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". |
  