Use case estimation - the devil is in the detail

Mission critical and complex software projects habitually exceed budget expectations significantly. Dependable cost estimates are often required by customers long before detailed analysis and design activities would produce this information during a project. A number of estimation methodologies have evolved to produce reliable cost information at an early stage in the software life-cycle, however estimation continues to be a contributor to budget blowouts. Contemporary techniques for costing requirements described as use cases are increasingly challenged as the size and complexity of the system expands. In addition, use case representations of requirements fail to directly map into structures used by project managers, leading to ongoing comparisons of individual costs that are subjective and often unrepresentative of final project expenditure. A large and complex system development project is described to demonstrate some of these problems and a potential solution is proposed to improve use case estimation.