A method for processing information includes receiving a set of records, which include a plurality of fields containing data regarding respective items, and selecting a field that occurs in all of the records and contains multiple terms in each of the records. At least first and second terms that occur among the terms in the selected field in the records are identified, such that the records are partitioned into at least first and second respective subsets by occurrences of the at least first and second terms in the selected field. Responsively to partitioning of the records by the occurrences, it is determined that the at least first and second terms correspond to at least first and second different values of an attribute of the items. The data are classified according to the values of the attribute.