Once the WBS has been created the project manager and the project team can begin to create an estimate of how long the project work will actually take and what resources are required to complete the project work. Resources are generally thought of as people and the roles they play within a project (think network engineer, mechanical engineer, and architect). Resources, however, are also things like materials, equipment, and facilities that the project needs in order to be successful.
When the project manager first begins to estimate the duration of the project she should always look for historical information. If a project of this type has been completed before historical information will be the best input to the project duration estimating process. This approach is also known as analogous estimating – where the project manager is creating an analogy between the two projects. When using an analogous estimate the projects don't, and often won't, be identical. It's an estimate based on projects that have completed in the past.
Real World Note: An analogous estimate is also known as top-down estimating. It's not a very reliable estimate, but it's a quick way to estimate the project duration.
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