This code system http://hl7.org/fhir/risk-probability defines the following codes:

CodeDisplayDefinition
negligibleNegligible likelihoodThe specified outcome is exceptionally unlikely.
lowLow likelihoodThe specified outcome is possible but unlikely.
moderateModerate likelihoodThe specified outcome has a reasonable likelihood of occurrence.
highHigh likelihoodThe specified outcome is more likely to occur than not.
certainCertainThe specified outcome is effectively guaranteed.