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Table 1 Hierarchy of levels of evidence from published papers, and grades of recommendation based on this hierarchy, as used by the Scottish Intercollegiate Guidelines Network [6]

From: Breaking new ground: challenging existing asthma guidelines

Hierarchy of levels of evidence from published papers
   1++ High quality meta-analyses, systematic reviews of randomized controlled trials (RCTs), or RCTs with a very low risk of bias
   1+ Well conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias
   1- Meta-analyses, systematic reviews of RCTs, or RCTs with a high risk of bias
   2++ High quality systematic reviews of case control or cohort studies
  High quality case control or cohort studies with a very low risk of confounding, bias, or chance and a high probability that the relationship is causal
   2+ Well conducted case control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal
   2- Case control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relationship is not causal
   3 Non-analytic studies, eg, case reports, case series
   4 Expert opinion
Grades of recommendation
   A At least one meta-analysis, systematic review, or RCT rated as 1++ and directly applicable to the target population, or
  A systematic review of RCTs or a body of evidence consisting principally of studies rated as 1+, directly applicable to the target population and demonstrating overall consistency of results
   B A body of evidence including studies rated as 2++, directly applicable to the target population and demonstrating overall consistency of results, or
  Extrapolated evidence from studies rated as 1++ or 1+
   C A body of evidence including studies rated as 2+, directly applicable to the target population and demonstrating overall consistency of results, or
  Extrapolated evidence from studies rated as 2++
   D Evidence level 3 or 4, or
  Extrapolated evidence from studies rated as 2+
  1. Reprinted with permission from the Scottish Intercollegiate Guidelines Network (SIGN) [6].