Good: Meets all criteria: comparable groups are assembled initially and maintained throughout the study; follow-up at least 80 percent; reliable and valid measurement instruments applied equally to the groups; interventions clearly defined; important outcomes are considered; and appropriate attention to confounders in analysis. In addition, for RCTs, intention-to-treat analysis is used.

Fair: Generally comparable groups assembled initially but some question remains whether some (although not major) differences occurred in 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 confounders are accounted for. Intention-to-treat analysis is done for RCTS.

Poor: Groups assembled initially are not close to being comparable or maintained throughout the study; measurement instruments are unreliable or invalid or not applied at all equally among groups; outcome assessment not masked; and key confounders are given little or no attention. For RCTs, no intention-to-treat analysis.

Good: Appropriate ascertainment of cases and nonbiased selection of case and control participants; exclusion criteria applied equally to cases and controls; response rate equal to or greater than 80 percent; diagnostic procedures and measurements accurate and applied equally to cases and controls; and appropriate attention to confounding variables.

Fair: Appropriate ascertainment of cases and controls and exclusion criteria applied equally to cases and controls, and without major apparent selection or diagnostic work-up bias; response rate less than 80 percent; or attention to some but not all important confounding variables.

Poor: Major selection or diagnostic work-up biases; response rates less than 50 percent; or inattention to confounding variables.