APPENDIX 3

Levels of Evidence/Certainty Regarding Net Benefit

US Preventive Services Task Force63Canadian Task Force on Preventive Health Care64
Level of CertaintyDescriptionLevel of EvidenceDescription
HighEvidence usually includes consistent results from well-designed, well-conducted studies in representative primary care populations. These studies assess the effects on health outcomes. This conclusion is unlikely, therefore, to be strongly affected by the results of future studies.IEvidence obtained from at least 1 properly randomized trial
II-1Evidence obtained from well-designed controlled trial without randomization
ModerateThe evidence is sufficient to determine the effects of the service on health outcomes, but confidence in the estimate is constrained by factors such asII-2Evidence obtained from well-designed cohort or case-controlled analytic studies, preferably from >1 center or research group
    Number, size, or quality of studiesII-3
    Inconsistency of findings across studiesEvidence obtained from comparisons between times and places, with or without the intervention; dramatic results in uncontrolled experiments could also be included in this category
    Limited generalizability of findings
    Lack of coherence in the chain of evidence
As more information becomes available, the magnitude or direction of the observed effect could change, and this change may be large enough to alter the conclusion
LowThe available evidence is insufficient to assess effects on health outcomes. Evidence is insufficient because ofIIIOpinions of respected authorities based on clinical experience, descriptive studies, or reports of expert committees
    Limited number or size of studies
    Important flaws in study design/methods
    Inconsistency of findings across studies
    Gaps in the chain of evidence
    Findings not being generalizable
    Lack of information on important health outcomes
More information may allow estimation of effects of health outcomes.