This chapter discusses the varying nature and form of cost data needed through the cost-planning process. It stresses that wherever possible construction economists will prefer to use their own cost data sources as collected and classified within their own organisations. The chapter explains the nature, purpose and form of elemental cost analyses but a typical functional element is that of a roof which provides the same function in a building independent of quantity, configuration or specification. It identifies the reasons for poor accuracy in tender bid prediction and consequently the reasons why the estimating methods that were deployed at that time were no longer working successfully. The majority of building contracts, especially those contracted by central and local governments, were lump sum firm price contracts, based on a standard form of contract, together with the architect's working drawings and bills of quantities. The modelling facilitated statistical analysis of data to provide more confidence in the cost advice being produced.