Eligibility criteria for studies need to be established prior to the analysis.

The researcher should base the inclusion/exclusion criteria on the design aspects of the trials, the patient populations, treatment modalities, etc. that are congruent with the objectives of the overview. Looking across a variety of studies this process can get quite complicated.

Although subjective, some researchers grade the selected studies according to quality and may weight the studies accordingly in the analysis. One such example of a quality rating of randomized trials is the Jadad scale (Jadad et al. Assessing the quality of reports of randomized clinical trials: Is blinding necessary? Controlled Clinical Trials , 1996; 17: 1-12). Here are the questions that are asked as a part of this scale along with the scores that are associated with these answers:

 Is the study described as randomized?

• No, or yes but inappropriate method (0 points)
• Yes but no discussion of method (1 point)
• Yes and appropriate method (2 points)

 Is the study described as double blind?

• No, or yes but inappropriate method (0 points)
• Yes but no discussion of method (1 point)
• Yes and appropriate method (2 points)

 Is there a description of withdrawals/dropouts?

• No (0 points)
• Yes (1 point)