This code system http://hl7.org/fhir/certainty-type defines the following codes:

Code Display Definition
Overall Overall certainty Overall certainty of evidence (quality of evidence).
RiskOfBias Risk of bias methodologic concerns reducing internal validity.
Inconsistency Inconsistency concerns that findings are not similar enough to support certainty.
Indirectness Indirectness concerns reducing external validity.
Imprecision Imprecision fuzzy or wide variability.
PublicationBias Publication bias likelihood that what is published misrepresents what is available to publish.
DoseResponseGradient Dose response gradient higher certainty due to dose response relationship.
PlausibleConfounding Plausible confounding higher certainty due to risk of bias in opposite direction.
LargeEffect Large effect higher certainty due to large effect size.
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