1. Apparatus for assisting in the optimisation of a complex, hierarchical data structure comprising a plurality of elements each having data dependency upon other elements within the hierarchy, the apparatus comprising: database means for maintaining a data set incorporating information relating to a plurality of dependency structure matrices, each dependency structure matrix corresponding to one or more hierarchical levels in the data set; display means for displaying, to the user, a representation of said dependency structure matrices; user input means for effecting the change of a data dependency of a selected element at a selected level in said hierarchy; and data manipulation means for effecting consequential changes in the data set representing said changes in dependencies of elements throughout the hierarchical structure.