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Levels of Evidence

Each recommendation in the 2008 update of the Canadian Best Practice Recommendations for Stroke Care was evaluated against several criteria: the strength of the available research evidence to support the recommendation, the degree to which the recommendation drives system change or processes of care delivery, and the overall validity and relevance as a core recommendation for stroke care across the continuum.

The levels of evidence were determined through a structured ranking system that measured the strength of the results in a clinical trial or research study. The design of the study (such as a case report for an individual patient or a randomized double-blind controlled clinical trial) and the end points measured (such as survival or quality of life) affect the strength of the evidence.

The various types of study designs, in descending order of strength, include the following:

  • Randomized controlled clinical trials (double-blinded or non-blinded): This is considered the gold standard of study design.
  • Meta-analyses of randomized studies: Such analyses offer a quantitative synthesis of previously conducted studies. The strength of evidence from a meta-analysis is based on the quality of the conduct of individual studies. Meta-analyses of randomized studies are placed in the same category of strength of evidence as are randomized studies.
  • Nonrandomized controlled clinical trials.
  • Case series: Population-based, consecutive series, consecutive cases (not population-based) or nonconsecutive cases.

These clinical experiences are the weakest form of study design, but often they are the only information available.

Several rating systems have been used by guideline developers internationally to evaluate the strength of the evidence for their recommendations. These systems vary in the nomenclature used (alphabetical versus numeric), but there is usually reasonable equivalence in the definitions across the levels of evidence. Each best practice recommendation included in this document provides the level of evidence for the recommendation, and cites the core reference guideline(s) that was adapted or that contributed most to the wording of the recommendation (see Table 1 of the main document for definitions of abbreviations used for this purpose). Refer to the master reference list for a detailed list, including website addresses, of the core reference guidelines.

Evidence table: Summary of definitions for levels of evidence reported in this document*

Grade  Criteria
AStrong recommendation. Evidence from randomized controlled trials or meta-analyses of randomized controlled

trials. Desirable effects clearly outweigh undesirable effects, or vice versa.
BSingle randomized controlled trial or well-designed observational study with strong evidence; or well-designed

cohort or case–control analytic study; or multiple time series or dramatic results of uncontrolled experiment.

Desirable effects closely balanced with undesirable effects.
CAt least one well-designed, nonexperimental descriptive study (e.g., comparative studies, correlation studies, case

studies) or expert committee reports, opinions and/or experience of respected authorities, including consensus

from development and/or reviewer groups.
 *Based on Guyatt GH, Cook DJ, Jaeschke R, et al. Grades of recommendation for antithrombotic agents: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th edition). [published erratum in Chest 2008;34:47]. Chest 2008;133(6 Suppl):123S-131S.