1. A computer system for calculating a score for a separately accountable business unit, the score being indicative of a level of unaccounted for external environmental cost of economic activities of the separately accountable business unit, the system comprising: accessing device for accessing accounts data indicative of recognised costs for the separately accountable business unit; and an accounts processor configured for processing the accounts data by adding external environmental costs to the recognised costs, and configured for calculating the score for the separately accountable business unit using the processed accounts data.