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Table 1 Evidence levels were classified in accordance with the SIGN methodology

From: Proposed guidelines for the diagnosis and management of methylmalonic and propionic acidemia

Evidence level Criteria
1++ High quality meta-analyses, systematic reviews of randomized control trials (RCTs), or RCTs with a very low risk of bias.
1+ Well conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias.
1 Meta-analyses, systematic reviews or RCTs, or RCTs with a high risk of bias.
2++ High quality systematic reviews of case-control or cohort studies or high quality case-control or cohort studies with a very low risk of confounding bias, or chance and a high probability that the relationship is causal.
2+ Well conducted case-control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal.
2+ Case-control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relationship is not causal.
3 Non-analytic studies, e.g. case reports, case series.
4 Expert opinion.