2.3. Study quality assessment and data synthesis

The evidence was summarized and systematically assessed using standard abstraction forms. The quality of each individual piece of evidence was assessed using the grading system developed by the United States Preventive Services Task Force [19]. We focused on several study factors when assessing quality, including study design, diagnostic criteria for MS, assessment of contraceptive use, outcome assessment, adjustment for potential confounders, timing of contraceptive use relative to outcome assessment and participation and follow-up (FU) rates. We did not compute summary measures of association due to heterogeneity across the included studies related to study population, study design, classification of exposure and outcomes reported.