Good: Meets all criteria outlined below.

Fair: Generally comparable groups are assembled initially but some question remains whether some (although not major) differences occurred with follow-up; measurement instruments are acceptable (although not the best) and generally applied equally; some but not all important outcomes are considered; and some but not all potential covariates are accounted for. Intention to treat analysis is performed.

Poor: Studies will be graded “poor” if any of the following flaws exists: groups assembled initially are not close to being comparable or maintained throughout the trial; unreliable or invalid measurement instruments are used or not applied at all equally among groups (including not masking outcome assessment); and key covariates are given little or no attention. Intention to treat analysis is lacking.

Criteria

• Initial assembly of comparable groups:
  • For RCTs: potential covariates appropriately distributed
  • For cohort studies: potential confounders controlled
• Maintenance of comparable groups ≈ < 20% loss to follow-up in each arm
• Measurements equal, reliable, and valid
• Interventions comparable and clearly defined
• All important outcomes considered
• Analysis:
  • For RCTs: intention-to-treat, covariate adjustment
  • For cohort studies: adjustment for potential confounders for cohort studies
• Other aspects of analyses appropriate (e.g. missing data, sensitivity analyses)