What is Decomposition
Decomposition as defined in PMBOK (5th Edition) is a tool and technique used to divide and subdivide the project scope and deliverable into more smaller, manageable work packages. Once the scope is decomposed, it’ll be the lowest level of WBS (Work Breakdown Structure) which can be estimated and managed in terms of cost and schedule. The size of the work package after decomposition will depend on the complexity and size of the project. At the time of decomposing, activities such as assigning identification codes to each work package will take place.
There are few approaches followed to structure a WBS. The 2 most popular methods are;
- Top-down approach
- Bottom-up approach
As it’s shown in the above figure, the high level task ‘1’ has been broken down into 4 sub tasks (1.1, 1.2, 1.3 and 1.4) which are further subdivided into smaller work packages.
E.g.: 1.1 has been decomposed into 3 smaller work packages (1.1.1, 1.1.2 and 1.1.3) These work packages are smaller, manageable and can be estimated in terms of cost and schedule.
Decomposition will not be valid and possible for a task or a deliverable that will need to be accomplished and built far into the future. Therefore, project management team has to wait until that particular requirement / deliverable is confirmed, hence a details WBS can be developed. This function is sometimes referred to as ‘rolling wave planning’ (Source: PMBOK, 5th Edition)
In addition, when the project deliverables / activities are decomposed into smaller, manageable work packages, the project management team should be aware of decomposing it to a level which is useful for a valid estimation. If over-decomposed, estimating those work packages will not be possible and the effort put on for decomposition will be a waste. Since this is known to be a team work / activity, everyone on the project team is requested to participate in bringing up the most efficient WBS of the project.