Project managers must take cost estimates seriously if they want to deliver software projects within budget constraints. After developing a good list of resource requirements, project managers and their software development teams should develop several cost estimates for those resources. There are several different tools and techniques for making a good cost estimate. Software development project managers must prepare several types of cost estimates for most projects. Three basic types of estimates include a rough order of magnitude or ROM, a budget estimate, and a definitive estimate. A rough order of magnitude estimate provides an estimate of the cost of a project. An order of magnitude rough estimate may also be called a rough estimate, a rough estimate, a swag, or a broad scale. This type of estimation is done in the early stages of a software development project, or even before a project officially begins. Project managers and upper-level management use this type of estimation to help make project selection decisions. The time frame for this type of estimate is typically three years or more before the software project is completed. The accuracy of a rough order of magnitude estimate is typically twenty-five percent less or seventy-five percent more than the actual final cost of the project. In some cases, software development project managers will automatically double software development project estimates due to a history of cost overruns on information technology-based projects. A budget estimate is used to allocate money to an organization's budget. Many organizations develop budgets at least two years out. Budget estimates are made one to two years before the software project is completed. The accuracy of budget estimates is typically ten percent to twenty-five percent below the actual final cost of the project. A final estimate provides an accurate estimate of project costs. Final estimates are used to make many purchasing decisions where accurate estimates are required and to estimate the final costs of the software development project. As an example, if a software development project involves purchasing 50 copies of a compiler suite from an external vendor over the next 2 months, a definitive estimate would be necessary to help evaluate the contractors' proposals. suppliers and allocate funds to pay the chosen supplier. . Final estimates are made one year or less before the completion of the software development project..