1. A method for establishing mapping rules for mapping user codes (MR payer IDs) to prescription transaction data records, wherein each data record comprises at least a first, a second and a third data fields, the method comprising: analyzing a set of previously mapped data records that have been assigned MR payer IDs based on at least the values of the third data fields; determining unique combinations of first and second data field values present in the set of data records; for each unique combination of the first and second data field values, determining a MR payer ID that is frequently assigned to the corresponding data records, wherein the frequency of assignment is above a cutoff limit value; establishing a mapping rule that assigns the frequently mapped MR payer ID to all data records having the unique combination of the first and second data field values; Wherein a mapping rule is assigned only for the unique combinations of the first and second data field values that occur with a frequency greater than a cutoff limit.