When credible data from prognostic observational studies were not available, we used risk estimates from control groups of RCTs included in the meta-analyses (e-Figures 5 and 6). Assessing Quality of Evidence Based on the Grades of Recommendations, Assessment, Development, and Evaluation (GRADE) approach, quality of evidence (also known as certainty of evidence) is de ned as the extent to which our con dence in the effect estimate is adequate to support a recommendation.6,7 The quality of evidence is categorized as high (A level), moderate (B level), or low (includes very low) (C level).6,7 The rating of the quality of evidence re ects the strengths and limitations of the body of evidence and was based on the study design, risk of bias, imprecision, inconsistency, indirectness of results, and likelihood of publication bias, in addition to factors speci c to observational Using GRADEpro software (version 3.6), we generated tables to summarize the judgments of the quality of the evidence and the relative and absolute effects.13 The GRADE tables include Summary of Findings tables presented in the main text, and a more detailed version called Evidence Pro les presented in the online supplement. The evidence pro les also explicitly link recommendations to the supporting evidence. Drafting of Recommendations Following the GRADE approach, the strength of a recommendation is de ned as the extent to which we can be con dent that the desirable effects of an intervention outweigh its undesirable effects. The strength of recommendation was categorized as strong (grade 1) or 320 Evidence-Based Medicine [ 1 4 9 # 2 C H E S T F E B R U A R Y 2 0 1 6 ] weak/conditional (grade 2). 