The Mott MacDonald study has provided a measure of the typical optimism bias for the various project types identified.
The data collection process revealed difficulties with respect to gathering information on operating expenditure and benefits shortfall. Firstly, data on operating expenditure and benefits shortfall was broadly unavailable and, secondly, determining benefits shortfall was based on personal interpretation as benefits estimated at business case were not clearly defined.
The relative contributions to optimism bias by the project risk areas were successfully measured, although a large degree of best judgement was involved.
The Mott MacDonald study identified the critical project risk areas that need to be managed, by putting in place risk mitigation measures when developing a business case, to reduce the likely optimism bias. Also, optimism bias reduction is likely to be achieved at least in part through priced risk transfer and this should be taken into account in any analysis. The project risk areas that have not had an impact on optimism bias were effectively managed in the projects studied.