Table 1 |
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Levels of evidence for therapeutic interventions |
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| Level of evidence |
Therapy/prevention, aetiology/harm |
|
|
|
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| 1 |
A |
Systematic review (with homogeneity*) of randomised controlled trials |
| B |
Individual randomised controlled trial (with narrow confidence interval) |
|
| C |
All or none |
|
| 2 |
A |
Systematic review (with homogeneity) of cohort studies |
| B |
Individual cohort study (including low-quality randomised controlled trial [e.g. <
80% follow-up]) |
|
| C |
'Outcomes' research; ecological studies |
|
| 3 |
A |
Systematic review (with homogeneity*) of case–control studies |
| B |
Individual case–control study |
|
| 4 |
Case series (and poor-quality cohort and case–control studies) |
|
| 5 |
Expert opinion without explicit critical appraisal, or based on physiology, bench
research or 'first principles' |
|
|
|
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Modified from http://www.cebm.net/levels_of_evidence.asp webcite. *By homogeneity we mean a systematic review that is free of worrisome variations (heterogeneity) in the directions and degrees of results between individual studies. Not all systematic reviews with statistically significant heterogeneity need be worrisome, and not all worrisome heterogeneity need be statistically significant. |
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Meeran and Grocott Critical Care 2005 9:81 doi:10.1186/cc2932 |
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