5. Evaluating the Evidence


Each paper eligible for inclusion should be analysed using the criteria from the Oxford Centre for Evidence-Based Medicine Levels of Evidence reproduced below:

Level

Therapy/Prevention, Aetiology/Harm

1a

Systematic review (with homogeneity*) of randomised controlled trials

1b

Individual randomised controlled trial (with narrow confidence interval)

1c

All or none §

2a

Systematic review (with homogeneity*) of cohort studies

2b

Individual cohort study (including low quality randomised controlled trial; e.g., <80% follow-up)

2c

"Outcomes" research; ecological studies

3a

Systematic review (with homogeneity*) of case-control studies

3b

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"

 

*               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. Studies displaying worrisome heterogeneity should be tagged with a "-" at the end of their designated level.

§              Met when all patients died before the treatment became available, but some now survive on it; or when some patients died before the treatment became available, but none now die on it.

§§            By poor quality cohort study we mean 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 we mean 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.

 

Grades of Recommendation

A

consistent level 1 studies

B

consistent level 2 or 3 studies or extrapolations from level 1 studies

C

level 4 studies or extrapolations from level 2 or 3 studies

D

level 5 evidence or troublingly inconsistent or inconclusive studies of any level

 

"Extrapolations" are where data is used in a situation which has potentially clinically important differences than the original study situation.

From http://www.cebm.net/levels_of_evidence.asp#notes. Produced by Bob Phillips, Chris Ball, Dave Sackett, Doug Badenoch, Sharon Straus, Brian Haynes, Martin Dawes since November 1998.


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