Definition of the Different Classes of Evidence (CoE)
Articles on treatment
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Studies of therapy
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Class
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Risk of bias
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Study design
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Criteria
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I
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Low risk
Study adheres to commonly held tenets of high quality design, execution and avoidance
of bias
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Good quality RCT
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• Random sequence generation
• Allocation concealment
• Intent-to-treat analysis
• Blind or independent assessment for important outcomes
• Co-interventions applied equally
• F/U rate of 80%+
• Adequate sample size
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II
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Moderately low risk
Study has potential for some bias; study does not meet all criteria for class I, but
deficiencies not likely to invalidate results or introduce significant bias
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Moderate or poor quality RCT
Good quality cohort
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• Violation of one of the criteria for good quality RCT
• Blind or independent assessment in a prospective study, or use of reliable data[a] in a retrospective study
• Co-interventions applied equally
• F/U rate of 80%+
• Adequate sample size
• Controlling for possible confounding[b]
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III
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Moderately high risk
Study has significant flaws in design and/or execution that increase potential for
bias that may invalidate study results
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Moderate or poor quality cohort
Case-control
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• Violation of any of the criteria for good quality cohort
• Any case-control design
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IV
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High risk
Study has significant potential for bias; lack of comparison group precludes direct
assessment of important outcomes
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Case series
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• Any case series design
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a Outcome assessment is independent of healthcare personnel judgment. Reliable data
are data such as mortality or re-operation.
b Authors must provide a description of robust baseline characteristics, and control
for those that are unequally distributed between treatment groups.
Articles on prognosis or risk
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Studies of prognosis
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Class
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Risk of bias
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Study design
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Criteria
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I
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Low risk
Study adheres to commonly held tenets of high quality design, execution and avoidance
of bias
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Good quality cohort[a]
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• Prospective design
• Patients at similar point in the course of their disease or treatment
• F/U rate of ≥ 80%[b]
• Patients followed long enough for outcomes to occur
• Accounting for other prognostic factors[c]
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II
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Moderately low risk
Study has potential for some bias; does not meet all criteria for class I but deficiencies
not likely to invalidate results or introduce significant bias
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Moderate quality cohort
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• Prospective design, with violation of one of the other criteria for good quality
cohort study
• Retrospective design, meeting all the rest of the criteria in class I
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III
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Moderately high risk
Study has flaws in design and/or execution that increase potential for bias that may
invalidate study results
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Poor quality cohort
Good quality case-control or cross-sectional study
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• Prospective design with violation of 2 or more criteria for good quality cohort,
or
• Retrospective design with violation of 1 or more criteria for good quality cohort
• A good case-control study[d]
• A good cross-sectional study[e]
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IV
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High risk
Study has significant potential for bias; does not include design features geared
toward minimizing bias and/or does not have a comparison group
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Poor quality case-control or cross-sectional
Case series[d]
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• Other than a good case-control study
• Other than a good cross-sectional study
• Any case series[f] design
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a Cohort studies follow individuals with the exposure of interest over time and monitor
for occurrence of the outcome of interest.
b Applies to cohort studies only.
c Authors must consider other factors that might influence patient outcomes and should
control for them if appropriate.
d A good case-control study must have the all of the following: all incident cases
from the defined population over a specified time period, controls that represent
the population from which the cases come, exposure that precedes an outcome of interest,
and accounting for other prognostic factors.
e A good cross-sectional study must have all of the following: a representative sample
of the population of interest, an exposure that precedes an outcome of interest (e.g.,
sex, genetic factor), an accounting for other prognostic factors, and for surveys,
at least a 80% return rate.
f A case-series design for prognosis is one where all the patients in the study have
the exposure of interest. Since all the patients have the exposure, risks of an outcome
can be calculated only for those with the exposure, but cannot be compared with those
who do not have the exposure. For example, a case-series evaluating the effect of
smoking on spine fusion that only recruits patients who smoke can simply provide the
risk of patients who smoke that result in pseudarthrosis but cannot compare this risk
to those that do not smoke.
Determination of Overall Strength of Evidence (SoE)
After individual article evaluation, the overall body of evidence with respect to
each outcome is determined based on precepts outlined by the Grades of Recommendation
Assessment, Development and Evaluation (GRADE) Working Group and recommendations made
by the Agency for Healthcare Research and Quality (AHRQ). Qualitative analysis is
performed considering the AHRQ required and additional domains. The table below provides
an outline of the method used to determine the final SoE.
Strength of Evidence for Existing Systematic Reviews
Level of evidence ratings for Cochrane reviews and other systematic reviews are assigned
a baseline score of HIGH if RCTs were used, LOW if observational studies were used.
The rating can be upgraded or downgraded based on adherence to the core criteria for
methods, qualitative, and quantitative analyses for systematic reviews (there is a
reference/evaluation table for this).
The following four possible levels and their definition are reported:
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High: High confidence that the evidence reflects the true effect. Further research is
very unlikely to change our confidence in the estimate of effect.
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Moderate: Moderate confidence that the evidence reflects the true effect. Further research
may change our confidence in the estimate of effect and may change the estimate.
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Low: Low confidence that the evidence reflects the true effect. Further research is likely
to change the confidence in the estimate of effect and likely to change the estimate.
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Insufficient: Evidence either is unavailable or does not permit a conclusion.
All AHRQ “required” and “additional” domains[a] are assessed. Only those that influence the baseline grade are listed in table.
Baseline strength: Risk of bias (including control of confounding) is accounted for
in the individual article evaluations. High = majority of articles level I/II; low = majority
of articles level III/IV
Downgrade: Inconsistency[b] of results (1 or 2); Indirectness of evidence (1 or 2); Imprecision of effect estimates
(1 or 2); Sub-group analyses not stated apriori and no test for interaction (2)
Upgrade: Large magnitude of effect (1 or 2); Dose response gradient (1)
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Outcome
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Strength of evidence
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Conclusions and comments
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Baseline
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Downgrade
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Upgrade
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Outcome
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High
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Summary of findings
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High
Level I/II studies
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No
Consistent, direct, and precise estimates
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No
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Outcome
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Moderate
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Summary of findings
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Low
Level III studies
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No
Consistent, direct, and precise estimates
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Yes
Large effect
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Outcome
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Low
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Summary of findings
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High
Level I/II studies
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Yes (2)
Inconsistent
Indirect
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No
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a Required domains: risk of bias, consistency, directness, precision. Plausible confounding
that would decrease observed effect is accounted for in our baseline risk of bias
assessment through individual article evaluation. Additional domains: dose-response,
strength of association, publication bias.
b Single study = “consistency unknown.”
