Key points
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In the GRADE approach, both randomized trials (which start as high quality evidence) and observational studies (which start as low quality evidence) can be rated down if relevant evidence comes from studies that suffer from a high risk of bias.
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Risk of bias can differ across outcomes when, for instance, each outcome is informed by a different subset of studies (e.g. mortality from some trials, quality of life from others).
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Current systematic reviews are often limited in their usefulness for guidelines because they rate risk of bias by studies across outcomes rather than by outcome across studies.