"A relationship of two Quantity values - expressed as a numerator and a denominator." - HL7 FHIR, Ratio. This data type represents a Ratio, where the Low and High Limits are Physical Quantities. A Ratio is " A quantity constructed as the quotient of a numerator quantity divided by a denominator quantity." - HL7 V3 A Physical Quantity is "a dimensioned quantity expressing the result of measuring" - HL7 V3