• Were the sample definition and selection prospective or retrospective?
• Were inclusion/exclusion criteria clearly described?
• Were participants selected to be representative?
• Was there an attempt to balance groups by design?
• Were baseline prognostic characteristics clearly described and groups shown to be comparable?
• Were interventions clearly specified?
• Were participants in treatment groups recruited within the same time period?
• Was there an attempt by investigators to allocate participants to treatment groups in an attempt to minimize bias?
• Were concurrent/concomitant treatments clearly specified and given equally to treatment groups?
• Were outcome measures clearly valid, reliable, and equally applied to treatment groups?
• Were outcome assessors blinded?
• Was the length of followup adequate?
• Was subject attrition below an overall high level (<20 percent)?
• Was the difference in attrition between groups below a high level (<15 percent)?
• Did the analysis of outcome data incorporate a method for handling confounders such as statistical adjustment?

The rating of intervention studies encompassed three quality categories:

• Good studies meet all criteria; comparable groups were assembled initially and maintained throughout the study (followup at least 80 percent); reliable and valid measurement instruments were used and applied equally to the groups; interventions are spelled out clearly; all important outcomes were considered; appropriate attention was given to confounders in analyzing data.
• Fair studies had any or all of the following problems, but without the fatal flaws noted in the “poor” category below; comparable groups were assembled initially, but some questions remain about whether some (although not major) differences occurred with followup; measurement instruments were acceptable (although not the best) and were generally applied equally; some, but not all, important outcomes were considered; some, but not all, potential confounders were accounted for.
• Poor studies have any of the following fatal flaws; groups assembled initially were not close to being comparable or maintained throughout the study; unreliable or invalid measurement instruments were used or not applied at all equally among groups (including not masking outcome assessment); key confounders were given little or no attention.