Least risk of bias, results generally considered valid

Employ valid methods for selection, inclusion, and allocation of patients to treatment; report similar baseline characteristics in different treatment groups; clearly describe attrition and have low attrition; use appropriate means for preventing bias (e.g., blinding of patients, care providers, and outcomes assessors); and use appropriate analytic methods (e.g., intention-to-treat analysis)

Susceptible to some bias but not enough to necessarily invalidate results

May not meet all criteria for good quality, but no flaw is likely to cause major bias; the study may be missing information making it difficult to assess limitations and potential problems

Category is broad; studies with this rating will vary in strengths and weaknesses; some fair-quality studies are likely to be valid, while others may be only possibly valid

Significant flaws that imply biases of various kinds that may invalidate results; “fatal flaws” in design, analysis or reporting; large amounts of missing information; discrepancies in reporting; or serious problems with intervention delivery

Studies are at least as likely to reflect flaws in the study design or execution as the true difference between the compared interventions

Considered to be less reliable than higher quality studies when synthesizing the evidence, particularly if discrepancies between studies are present