1. A method for automating classification of a new data item when adding the new data item to an hierarchically organized; set of classified data items, wherein nodes of the hierarchy correspond to classes of data items, the method comprising the steps of: for a new data item requiring classification within the set of classified data items, identifying classification attributes of the new data item by reference to a set of classification attributes for the set of classified data items; calculating a conditional value representative of the randomness of distribution of classification attributes for data items within the set of classified data items, which value is conditional on the new data item being added to a first class at a particular level of the hierarchy, and repeating the step of calculating a conditional value for each sibling class of the first class at said particular level of the hierarchy wherein said conditional values are each conditional on adding the new data item to a different respective one of said sibling classes at said particular level of the hierarchy; comparing the conditional values to identify the lowest conditional value; and selecting the class having the lowest conditional value for classifying the new data item.