24 The same approach should be taken with estimating the length of time it will take to complete the capital works. In summary:
❑ Estimate the time taken to complete the capital works;
❑ Apply adjustments to these estimates, based on the best available empirical evidence relevant to the stage of the appraisal;
❑ Subsequently, reduce these adjustments according to the extent of confidence in the works duration estimates, the extent of management of generic risks, and the extent of work undertaken to identify and mitigate project specific risks; and,
❑ The estimates of works' duration, and the adjustments for optimism, should ideally be reviewed independently.
25 The application of optimism bias adjustments to works' duration should be reflected in a delay in the receipt of benefits. This will be shown in the net present value calculations. The appraisal period may need to be extended to reflect the expected delay in benefits' stream, but different periods should not usually be set for different options.