Quantity-significance and its application to construction project modelling

Quantity-significant items (QSI) are defined as those items whose quantities are greater than the mean quantity of similar items within one trade. It has been found that QSI's number about 30% of the items in a trade. They consistently contribute about 80% of the quantities and 80% of the value. This relationship, from which the principle of quantity-significance derives, has been used to develop new integrated cost and time models. These allow the prediction of the total project value and duration by using only a small fraction of the elements traditionally used in conventional methods. The models which are based upon quantity-significant work packages and operations QSWP and QSWO) have been developed and tested using historical projects of two sorts; reinforced concrete bridges and structural steel framed supermarkets. The accuracy of the models at detailed design stage was within 4% for both cost and time estimation. At feasibility stage, the models could predict the total cost to within 11%. The effect of using the models on the final account has also been considered. Three methods of dealing with variations have been proposed. Two of these methods have been shown to yield very accurate long term results. In the short term, individual errors of the order of 3.75% in 2/3 of cases have been predicted. Since the same elements (i.e. QSWO's) are used for both cost and time estimation, and because they are related to site activities, the detailed models can be used to provide an effective mechanism for control of site operations and feedback to estimators.