NASA categories of evidence

Evidence provided in the Human Research Program's various evidence reports, authors were urged to label the evidence according to "NASA's Categories of Evidence" to indicate whether evidence is from controlled experiments, is observational or based upon expert opinion. These categories were based upon an already used, familiar scale.

Broad "Experimental" Design Type	Silagy & Haines Lelvels of Evidence[1]	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".

References

1. Silagy, C; Haines, A (2001). Evidence Based Practice in Primary Care (PDF) (2nd ed.). London: BMJ Books.