Shocks such as the oil crisis and the macro-economic business cycle had a marked impact on some projects and the 1980s included significant economic and social changes. The current economic climate suggests that this cause of optimism bias may have reduced.
The optimism bias assessment does not consider the effect of tender and construction cost indices on capital expenditure. However, when appraising future and current projects, changes in indices can only be predicted and not guaranteed. If trends in the tender and construction cost indices are not taken into consideration, or not accounted for in the business case, then the behaviour of indices may influence the outturn costs of a project.