1. A method of mapping reference data, comprising: mapping a first reference value from a first set of reference data to a first ontology value in an ontology, wherein the first reference value characterizes one or more data values in a first database; mapping a second reference value from a second set of reference data to a second ontology value in the ontology, wherein the second reference value characterizes one or more data values in a second database; determining a first association between the first ontology value and the second ontology value; and based on the determined first association, and by use of one or more computer processors, determining a second association between the first value from the first set of reference data and the second value from the second set of reference data.