Hierarchy of strength of evidence used in NICE technology appraisals (strongest to weakest) (1)
Ia evidence from systematic reviews or meta-analysis of randomised controlled trials
Ib evidence from at least one randomised controlled trial
IIa evidence from at least one controlled study without randomisation
IIb evidence from at least one other type of quasi-experimental study such as a cohort study
III evidence from non-experimental descriptive studies, such as comparative studies, correlation studies, case-control studies and case series.
IV evidence from expert committee reports or opinions or clinical experience of respected authorities
There are slight variations on this classification structure (2)
Level of Evidence | Grading Criteria | Grade of Recommendation |
1a | Systematic review of RCTs including meta-analysis | A |
1b | Individual RCT with narrow confidence interval | A |
1c | All or none studies - met when all patients died before the Rx became available, but some now survive on it; or when some patients died before the Rx became available, but none now die on it. | B |
2a | Systematic review of cohort studies | B |
2b | Individual cohort study and low quality RCT | B |
2c | Outcome research study^ | C |
3a | Systematic review of case-control studies | C |
3b | Individual case-control study | C |
4 | Case-series, poor quality cohort* and case-control studies** | C |
5 | Expert opinion | D |
^ outcomes research is a public health research, which studies the end results of the structure and processes of health care system on the health, and well-being of patients and populations
* By poor quality cohort study - means one that failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same (preferably blinded), objective way in both exposed and non-exposed individuals and/or failed to identify or appropriately control known confounders and/or failed to carry out a sufficiently long and complete follow-up of patients
**By poor quality case-control study - means one that failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same (preferably blinded), objective way in both cases and controls and/or failed to identify or appropriately control known confounders
Reference:
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