Green Book supplementary guidance: optimism bias
Supplementary guidance to the Green Book on estimates for a project's costs, benefits and duration in the absence of robust primary evidence.
Project appraisers have the tendency to be over optimistic. Explicit adjustments should therefore be made to the estimates of a project’s costs, benefits and duration, which should be based on data from past or similar projects, and adjusted for the unique characteristics of the project in hand.
This guidance provides cost and time uplift percentages for generic project categories which should be used in the absence of more robust primary data.