4.2.4 Estimates
Estimates start when the project specification comes in. No estimate can ever be free of error as estimates involve personal judgements. Every project therefore involves an element of risk. The aim of estimating is to reduce the risk to within manageable proportions. Estimating costs involves a calculated judgement of what a project should cost if everything goes according to expectation. The estimate accuracy should depend on the estimator's confidence in estimating costs. Some companies ask estimators to provide a confidence rank 1 to 5 of the probable accuracy of the estimate.
Common types of estimates include:
- Ball Park Estimates ± 25%
Ball Park estimates are usually based on hazy information, these often provide a potential customer with a notion of how much the project will cost. Ball Park Estimates can never be used for fixed price tendering and at the best are ± 25% of the final costs.
- Comparative Estimates ± 15%
Comparative Estimates are made by comparing work to be done on the new project with work done on a similar project in the past. Comparative estimates work well when the type of projects received are fairly routine and achieve an accuracy around ± 15% of final costs. Therefore, comparative estimates should take into account as many contingencies as possible.
- Feasibility Estimates ± 10%
Feasibility Estimates are derived after significant project design is carried out and supplier quotes received. These estimates are used mainly in the construction industry and have a better than ± 10% accuracy.
- Definitive Estimates ± 5%
Definitive Estimates are usually derived from comparative or feasibility estimates and are updated as the project progresses. They form part of the cost reporting and control procedure and the accuracy improves with time as the known costs accumulate. The accuracy is within a ± 5% range and the actual project costs and the definitive estimates converge as the project ends.