Guideline: Estimation
This guideline provides some hints that can help the project manager to do estimations at the project level and at the iteration level.
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Related Elements
Main Description

Estimation Approaches

There are mainly two approaches to do estimation:

  • Top-down: where you estimate the total effort to develop the application, and then you distribute this effort to the lower level task using a defined formula, like COCOMO II.
  • Bottom-up: where you list the different tasks that need to be performed, you estimate them and by adding the different estimations you get the overall effort.

None of them is better than the other, and in fact they are complementary. Depending on the kind of estimation he will like to do the project manager will be using one or the other.

Estimating the overall effort and duration of the project

At the beginning of the project, the project manager does not have a lot of details on the scope of the project. He usually only knows the big picture. On such cases a top-down approach is more useful, as with what he knows about the scope he can arrive to estimate the total effort and he can then derive the ideal duration. This high-level estimation will also help him defining the duration of the phases, and can even also give him a first guess at the size of the team he needs to do the project.

Estimating an iteration

When he plan an iteration, the project manager will need to identify the different unitary tasks that must be performed to achieve the goals of the iteration to ensure that they will be done. Is then that the bottom-up approach can be successfully used, as he has yet the different tasks. A good approach to do this kind of estimation is to get it directly from the team member who will be doing the work.