A method for verifying the classification of a current data item within an hierarchical set of classified data items, wherein nodes of the hierarchy correspond to classes of data items and data items within the classes each have one or more classification attributes, and wherein the current item has been classified within a first class corresponding to a node within the hierarchical set of classified data items, the method comprising the steps of:
- calculating an initial value representative of the randomness of distribution 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 current data item being classified within a respective one of the set of sibling classes of the first class as an alternative to the first class, wherein sibling classes comprise the set of immediate descendant classes of a common ancestor class in the hierarchy;
- repeating the step of calculating a conditional value respectively for any other ones of said sibling classes, wherein said conditional values are each conditional on classifying the current data item within a different respective one of said sibling classes;
- and
- checking whether the initial value representative of the randomness of distribution of classification attributes is a lower value than each of the conditional values.