PMP Common Estimating Tools and Techniques

The project manager has several tools or techniques available for estimating activity duration, project costs, and preparing the project plan.
Analogous estimating, also known as top-down estimating: Relies on previous or similar projects to establish a broad or high level estimate. This type of estimate relies on expert judgment, lessons learned, and usually requires adjustments throughout the project.

Parametric estimating: Utilizes mathematical models such as the cost per square foot to build a house or commercial building.
Bottom-up estimating: Requires a detailed work breakdown structure, more time and possibly additional experience resources, but will produce a more definitive and more accurate estimate.
There are also several tools available for use by the project manager and team to provide the required estimates. These include project management software, commercial data bases, and internal organization check sheets and estimating models.
An important factor to remember is that estimation tools, like other niche tools, are usually not designed to be metrics tools. Any measurement or metrics type information obtained from an estimation tool is a by-product, or an extra feature. Over time, estimation tools are acquiring more and more utilitarian functions, some of which will support additional capabilities to produce measurement or metrics data or information. This is based on demands from the user community most specifically, the members of that community willing to pay for such capabilities. Some tools use the user's actual project results to adjust the estimating tool's database parameters to provide for a more accurate estimate with the next project. Periodically the maintenance users receive updates that reflect input from users' communities, so the tool continues to improve the accuracy in each new version.
Any estimating tools will have to be adjusted over time to accommodate new information and material cost (construction) if the integrity of the database is to be kept maintained. With software estimating tools, the development platform and the environment both have to be considered. On software development projects where much interaction is necessary with groups of people in various departments, another consideration is how well the interaction with these people is. If these departments are resistant to the project, then the estimate could take considerably longer than with departments where there is great cooperation.