A Work Breakdown Structure (WBS) is the sum of the project scope – all its requirements and deliverables – broken down into pieces for the purpose of managing activities. The WBS is a document that is created during the planning phase of a project and then used referentially throughout its lifespan. The Project Management Book of Knowledge (PMBOK) defines the WBS as “A hierarchical decomposition of the total scope of work to be carried out by the project team to accomplish the project objectives and create the required deliverables.”
To create a WBS, the project manager uses knowledge of the project to estimate duration, complexity, and cost and create work packages that can be managed. Like most things on a project, this is not done in isolation and may require expert advice, lessons learned from previous projects, and templates to guide and structure how the activities will be broken down then grouped. This process, called decomposition, can be performed using either a top-down or bottom-up approach and it will help establish a baseline for later comparison to ensure the project is on time, on budget, and of acceptable quality.
A supporting document to the WBS is the WBS dictionary and it contains detailed information about the work including:
- Descriptions and expectations
- Schedule of activities
- Key milestones
- Resources – both human and otherwise
- Estimates for time and cost
- Quality requirements
- Acceptance and audit criteria
- Coded identifiers (when appropriate)
The hierarchical nature of the WBS means that it is most commonly found in association with predictive project management methodologies, such as Waterfall. The project team uses expert knowledge to make determinations about effort and priority to group cards that are made ready to work only when the previous grouping of cards are complete.