Defining Project Breakdown Structures

By Sara Sumner, Vitech Applications Engineer

As a systems engineer, I have found that when working with complex projects, it is critically important to capture the relationships between the product and the functional work allocation. Capturing these relationships can help manage the uncertainty of impacts on the project when inevitable changes are introduced throughout the lifecycle and assist with the balance of resources needed to design and implement the system. This information is often defined in the project as the System Breakdown Structure (SBS), Product Breakdown Structure (PBS), Work Breakdown Structure (WBS) and Organization Breakdown Structure (OBS). Even though these hierarchical structures have been used extensively in Project Management and Systems Engineering communities, it is my experience that few people have a firm understanding of their definitions and how they should be used in conjunction to help manage the complexity of a project.

The System Breakdown Structure (SBS) is a logical decomposition of the system. When defining the SBS, it is important to consider how the system will be procured, how the system will be designed, what the system must do, and how you will manage the critical interfaces of the system. Oftentimes, the SBS will contain the system functionality (think functional requirements) in which the PBS items (delivered product) will satisfy. The SBS is used for documenting and communicating the functionality of the system. Once the project develops towards implementation, the SBS items will trace to the appropriate PBS items.

The Product Breakdown Structure (PBS) is created to ensure that all required products are considered during the planning process. The PBS begins with the final product at the top of the hierarchy and details the products that should be produced and delivered. Often times, the PBS provides physical components of the system, however it can also contain conceptual elements of the product, often including product hardware, software and documentation items and the relationships between them. The PBS is used for communicating the outcomes of a project, showing exactly what the project will deliver.

The Work Breakdown Structure (WBS) is a hierarchical decomposition of the work that is necessary to complete a project. When related to the PBS, the WBS addresses how each part of the system or product (identified in the PBS) is going to be completed. The WBS is used for overall planning of the project, assisting with the project management, cost estimation, project status reporting, and schedule and resource management. Usually, the WBS is created in the early stages of the project lifecycle, proceeding the development of the PBS.

The Organization Breakdown Structure (OBS), used in conjunction with the WBS, provides the organization structure for the project as it moves to completion. The OBS captures the organization/people that will perform the work identified in the WBS. When creating an OBS, it is important to consider the organization structure for the resources involved in the project, the team members associated with each organization, and openings within the organization. Once the OBS is defined, the WBS should be associated with the lowest level of project responsibility within the OBS hierarchy.

Each of the above described hierarchies, when used autonomously, can be used to breakdown the complexity of the project and assist with system and project analysis. However, once the SBS, PBS, WBS, and OBS are properly traced to each other, they will reveal necessary changes that need to be made to the project to provide a healthier balance between cost, resource, and design. They can also provide a powerful means of assessing impacts on the system when inevitable changes are introduced throughout the project lifecycle.

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