Project success is often internally measured and reported as performance against an agreed budget. Ideally such comparisons would be made based on linking approved budget to scope of work delivered, such as via the Earned Value Technique. This is frequently not done and in fact budgets are often adjusted as the need arises. To avoid errors in the benchmark study that would occur if current budget data was used this study adopts the final approved budget prior to going to market as the milestone.
This particular metric is the one adopted in the previous studies by Mott MacDonald, the NAO and Fitzgerald and thus it is useful as a comparator. However, it could be argued that this particular period simply gives an indicator of the accuracy of the budget estimate and that differences in the answer may be attributed to different levels of optimism at budget stage due to the rigour of the estimate. To avoid problems associated with optimism bias at budget approval it is considered comparisons based on the full period are more appropriate as optimism bias is reduced.