The estimation methodologies commonly dealt with in literature and used in current practice can be included in two general categories: conceptual and deterministic. In the conceptual estimation methods (or synthetic) we use calculation models based on reports, inferred or statistical, between the costs and the comparison parameters used (generally extrapolated from the design) and these most often require a non-negligible effort in the preliminary collection of data to be used in the analysis. The deterministic (or analytical) estimation methods, on the other hand, require a high level of precision in the determination of quantities, unit prices and scope definition.
The choice of one method of analysis over another is most often determined by the state of definition and accuracy of the design. In the initial phases of the project life cycle, when the level of in-depth design is still modest, synthetic estimation methods will be used while, on the contrary, when the level of detail of the design will become significant, it will be preferable to use the methods of analytical estimates.
There is a precise correlation between the level of in-depth design, the accuracy of the economic assessment and the effort required to prepare the estimate.