The two elements of a tender sum are the cost estimate and the mark-up. Variations in tenders can be attributed to inaccuracy in the estimate and variations in the applied mark-up. The weaknesses of current published literature are that the causes of the inaccuracy in the estimate or the variability in a mark-up are not defined in such a way that they can be analysed and evaluated. The inaccuracy of a cost estimate results from combining the inaccuracy of its constituent elements. Thus the inaccuracy of each element needs to be evaluated. The consequences of estimating inaccuracy and mark-up variability are inadequately described in current literature, which tends to assume that mark-up variability is negligible. This research addresses these weaknesses.