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Project Management Estimation Methods – Buttom Up Estimation

Posted by mgocean on May 28, 2009 in Estimation, Project Management |

Bottom-up estimation is based on a breakdown of the project into individual elements, each of which is estimated separately. Project elements could consist of individual requirements, tasks, or deliverables from a work breakdown structure, or counts of items such as classes, program modules, or test cases. The estimates for the individual elements are then compiled to get an estimate for the entire project.

 

In the Bottom-Up Method, estimators start with a detailed list of project tasks and separately estimate the resources (effort, duration, materials, and consumables) needed to perform each task (or a group of related tasks). Then they add the results together to produce the total for the entire project. Planners arrange the tasks in a logical network to explicitly handle constraints on task sequencing, the available resources, the total cost, and total time.

 

The Bottom-Up Estimation process is as follows;

  1. Define project tasks (the WBS based on architecture and life cycle).
  2. Estimate resources required for each task.
  3. Identify milestones by type:
    1. Customer directed
    2. Process related
    3. Management directed
  4. Assign dates to each milestone (based on master schedule).
  5. Construct and verify the task network (task dependencies).
  6. Assign resources to tasks.
  7. Estimate task duration
  8. Compute the critical path, near-critical paths, and evaluate reasonableness.
  9. Compare calculated dates with required dates in the master schedule.
  10. Adjust the task network to achieve required dates:
    1. Reduce the critical path if possible (repeat Steps 2, 6, 7, 8).
    2. If not feasible to achieve imposed milestones, renegotiate them with customer.
  11. Evaluate risks (amount of float and number of near-critical paths).
  12. Tune the network (optional):
    1. Level the resources to the extent possible using the available total float.
    2. Balance the task durations to the extent possible.

 

 

(Stutzke R. D., 2005)

Bottom-up estimation provides a more detailed and accurate basis for estimation, because it deals with low-level components. This method also supports project tracking more directly than other methods because its estimates usually address each activity within each phase of the software development lifecycle.

As all of the estimation methods, bottom-up estimation method has disadvantages; the amount of effort to prepare a bottom-up estimate is quite significant. Since the work breakdown structure tasks may be assigned to various groups and may not be clear, some activities can be included multiple times. Another disadvantage is when tasks are estimated inflated, the total project estimate may be dramatically misleading. If the estimate is done before the requirements are clarified it will give wrong information.

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