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Level III Opinions of respected authorities, based on clinical experience,
descriptive studies, or reports of expert committees.
U.S. Preventive Services Task Force system 78
LEVEL OF
STUDY DESIGN AND CRITICAL APPRAISAL
EVIDENCE
Level A Consistent Randomized Controlled Clinical Trial, cohort study, all or
none*, clinical decision rule validated in different populations
Level B Consistent Retrospective Cohort, Exploratory Cohort, Ecological Study,
Outcomes Research, case-control study; or extrapolations** from level A
studies
Level C Case-series study or extrapolations from level B studies
Level D Expert opinion without explicit critical appraisal, or based on physiology,
bench research or first principles
* the all or none principle is met when all patients died before the Rx became available, but some now
survive on it; or when some patients died before Rx became available, but none died on it
** “Extrapolations” are where data is used in a situation which has potentially clinically important differences
than the original study situation. Thus, the quality of evidence to support a clinical decision is a combination
of the quality of research data and the clinical ‘directness’ of the data 79
Oxford centre for Evidence-based medicine 80
QUALITY OF EVIDENCE DEFINITION
High Further research is very unlikely to change our confidence in
the estimate of effect
Moderate Further research is likely to have an important impact on our
confidence in the estimate of effect and may change the
estimate
Low Further research is likely to have an important impact on our
confidence in the estimate of effect and is likely to change the
estimate
Very Low Any estimate of effect is very uncertain
STRENGTH OF
DEFINITION
RECOMMENDATION***
Strong The desirable effects of an intervention clearly outweigh the
undesirable effects, or clearly do not
Weak The desirable and undesirable effects are closely balanced
***Factors that affect the strength of a recommendation: quality of evidence, uncertainty about the balance
between desirable an undesirable effects, uncertainty or variability in values and preferences, uncertainty
about whether the intervention represents a wise use of resources
Grade Working Group 81
78 http://www.ahrq.gov/clinic/3rduspstf/ratings.htm (hentet 14-02-2009)
79 Atkins D, Best D, Briss PA, et al (2004). "Grading quality of evidence and strength of recommendations".
BMJ 328 (7454): 1490. doi:10.1136/bmj.328.7454.1490. PMID 15205295. (hentet 14-02-2009)
80 Oxford Centre for Evidence-based Medicine: Levels of Evidence and Grades of Recommendation
http://www.cebm.net/index.aspx?o=1025 (hentet 14-02-2099)
81 Gordon HG, et al (2008). ” GRADE: an emerging consesus on rating quality of evidence and strength of
recommendations”. BMJ 336: 924-926 http://www.bmj.com/cgi/content/full/336/7650/924 (hentet 14-02-
2009)
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