Work Breakdown Structure (WBS): Top-down or Bottom-up?

Project Managers are always talking about Work Breakdown Structure (WBS). What is WBS and why is it needed? Simply put it is a hierarchical depiction of all tasks that must be done to complete a project. The tasks at the lowest hierarchical level define unit(s) of work that can be unambiguously defined and whose time, cost and resource requirements can be accurately computed.

There are essentially two ways to create a Work Breakdown Structure – the top-down or the bottom-up approach.

  • The top-down approach, in my opinion, generates a complete and more accurate WBS. In this approach, the WBS is derived by decomposing the overall project into sub-projects or lower-level tasks. This decomposition is based on general project characteristics and not on detailed design elements. The decomposition continues until the tasks or work units reach a level where they can be accurately defined and estimated. This approach is by far more popular than the bottom-up approach. Why? Because it is more logical! It is only rational to first define a solution to a problem and then dissect the solution into the steps required to implement them. The natural working of a human mind is to first start with a broad approach to a problem and then iteratively narrow it down to specifics.
  • The bottom-up approach on the other hand is more akin to a brain-storming exercise where team members are asked to make a list of low-level tasks needed to complete the project. In many instances this can turn quite chaotic if the tasks identified by the team are not all at the same level. It can also be time consuming to ensure that all tasks at a given level have been completely identified. This approach is resource intensive since it assumes that all members of the team have sufficient domain knowledge and a complete understanding of the project requirements in order to be able to identify and integrate tasks at different levels. The biggest disadvantage that I have found in bottom-up estimating is that almost always more than a few low-level tasks are inadvertently omitted because team members are either not knowledgeable or sensitive to all parts of the project. I do not recommend the bottom-up approach unless the WBS is created by a group of experts who have a very detailed knowledge of the project and its decomposed elements.
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