1. Apparatus arranged to receive data files from data sources and to categorise said data files to facilitate searching in response to user-requests, wherein said data files contain unspecified high value items, comprising identifying means configured to identify occurrences of unspecified candidate items in preferred contexts for a preferred specified category and to identify occurrences of unspecified candidate items in non-preferred contexts; and processing means configured to process said preferred occurrences with said non-preferred occurrences for each candidate item and to select a candidate item as a high value item.