The PMBOK® Guide, Seventh Edition defines a Work Breakdown Structure (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".
While this definition provides a good understanding of what a WBS is, it doesn't provide guidance on how to organize the structure of a WBS. This is helpful as it provides flexibility for teams to determine what is the most appropriate means of doing so for the context of their project.
Although there is an infinite variety of projects, I've found three approaches used by most teams who develop WBS's:
- Deliverable-oriented - each of the top-level components of the WBS represents a key deliverable, and the subsequent levels focus on the decomposition of those deliverables to an appropriate level of detail
- Phase-oriented - each of the top-level components of the WBS represents a phase or stage in the life cycle of the project, and the subsequent levels focus on the decomposition of the outputs of each of those phases or stages
- Responsibility-oriented - each of the top-level components of the WBS represents a contributing stakeholder to the project's delivery, and the subsequent levels focus on the decomposition of the outputs produced by each of those stakeholders
A deliverable-oriented approach helps to focus the team on everything required to complete each deliverable without worrying at that stage about who does it or when it would be done. This can work well for projects where the full scope of a project can be decomposed early on, but can be challenging when a rolling wave approach is taken unless there is a clear identification of planning packages which need to be elaborated at a later date.
A phase-oriented approach works well with a rolling wave approach as the team only needs to focus on what will be completed in the upcoming phase. However, there is the risk that the team might miss key scope elements for specific deliverables as they won't be focusing on decomposing a single deliverable at a time.
A responsibility-oriented approach works well when there are multiple stakeholders and there is the need to have a clear segregation of responsibility for planning and execution between them. It suffers from the same risk as the phase-oriented approach, but more severe as there is the potential for deliverable sub-components to be assumed by each stakeholder to be the responsibility of another.
I ran a poll in PMI's LinkedIn Project, Program and Portfolio discussion group as well as the ProjectManagement.com community to gauge the distribution of usage of these three approaches. Out of the combined 141 responses, 57% were using a deliverable-oriented breakdown, 28% used a phase-oriented approach and 9% used a responsibility-oriented one. The remaining 6% indicated they used another approach but when I reviewed the comments those participants had submitted, it seemed that they were just using a combination of these methods rather than something entirely new.
While the WBS is commonly-used tool with projects following a predictive life cycle, this doesn't mean that those following an adaptive one can't benefit from it. If we consider a user story map, it is just a WBS constrained to two to four levels and developed in a progressively elaborated manner. With a story map, the structure might be persona-oriented or theme/capability-oriented.
Regardless of how your team chooses to decompose the scope of work on their projects, a WBS or a variant thereof should be considered as a valuable tool in their toolboxes.
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