Cost estimates made early in the life of a project are necessarily less precise than later ones.
A good estimate has four attributes: it should be close to the true value; it should be unbiased, with successive estimates not deviating consistently from the true value in a single direction; it should be well calibrated, meaning that the range of the cost estimate's uncertainty interval frequently includes the true value; and it should be reliable, in the sense that an expert's estimates can be repeated by others.146
For an estimate to have these attributes, the estimator needs two things: adequate data, and a clear understanding of how to use it in constructing the estimate. But, as the following sections explain, the data available in Australia is far from adequate, and there are shortcomings in the official guidance on cost estimation.
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146. DIRDC (2018, pp. 16-17).