1. A method for processing information, comprising: receiving a set of records, which comprise a plurality of fields containing data regarding respective items; selecting a field that occurs in all of the records and contains multiple terms in each of the records; identifying at least first and second terms that occur among the terms in the selected field in the records, 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; determining, responsively to partitioning of the records by the occurrences, that the at least first and second terms correspond to at least first and second different values of an attribute of the items; and classifying the data according to the values of the attribute and outputting the classified data.