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

June 16, 2011 | Author: PM Hut | Filed under: Project Management for Beginners, Project Plan Development

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

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.

Dr. Samuel Prasad is a renowned global technology manager with a 15-year track record in helping companies on their implementation of strategic plans and programs related to technology projects for major media, entertainment, data warehousing and financial companies in the U.S., China, Europe and India. His domain expertise extends into areas of financial transaction processing, mobile, wireless, RFID, online media, casual gaming and business intelligence. Sam is a certified Project Management Professional (PMP) and a Certified Software Quality Engineer (CSQE). Sam has a Ph.D. in Robotics & Computer Science from the Stevens Institute of Technology (USA), and a master’s degree in Computer Science from the Indian Institute of Technology. Dr. Prasad can be reached at Intelligent Software Systems. Blog: http://blog.prasads.com/ - Email: blog@prasads.com - Telephone: 631-368-8130

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3 people have left comments

Samuel,
Thank you for this. I agree with your comments, and would add that bottom up works best on simpler and less structured projects. However, combining the approaches as I describe in my blog (like attached) allows a greater level of confidence for new PMs and inexperienced teams, who are not wholly confident about the logical deconstruction that top-down involves. Bottom-up sometimes brings missed elements to our awareness.
I also favour working bottom up when applying resources and budget to my WBS to create a CBS.
Mike

Mike Clayton wrote on June 16, 2011 - 1:47 pm | Visit Link

Samuel,

I’d like to get your opinion on the statement that work package should represent a deliverable or, at least, something mesurable but not a task, as task decomposition should come right after WBS creation.
Don’t you think this approach could also help for bottom-up ?
Pierre

P.E. Pernet wrote on June 17, 2011 - 4:23 am | Visit Link

Another weakness of the bottom-up approach is the difficulty in identifying task dependencies and potentially linked work. In either strategy, I think the use of a whiteboard (real or virtual) to graphically represent the WBS as it’s being constructed is immensely valuable.

Joe MacNish wrote on June 20, 2011 - 1:41 pm | Visit Link

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