(i)  Works Duration Optimism Bias

The actual works duration is compared to the works duration estimated at outline BC and contract award. The works duration refers to the implementation stage of the project, including design, mobilisation and construction. The works duration optimism bias can be represented as follows:

The measured optimism bias does not give any indication of whether the project was delivered on time, but only reflects the extent to which the works duration had increased. The time lines shown below give an indication of how works duration optimism bias is determined. If the implementation stage started early and finished on the expected date, the works duration optimism bias will show an increase in works duration (i.e. be positive), but the project should not be considered as having been delivered late. If the works started two weeks late and finished two weeks late (i.e. works duration actual = works duration estimated), the optimism bias measured will be 0%. However, this measure will fail to show that the project was delivered later than expected.

Figure 2  Estimated Project Time Line versus Actual Project Time Line