Work Breakdown Structure
A work breakdown structure is the cornerstone of every program because it defines in detail the work necessary to accomplish a program’s objectives. For example, a typical WBS reflects the requirements to be accomplished to develop a program, and it provides a basis for identifying resources and activities necessary to produce deliverables. A WBS is also a valuable communication tool between systems engineering, program management, and other functional organizations because it provides a clear picture of what has to be accomplished by decomposing the project scope into finite deliverables. Accordingly, it is an essential element for identifying activities in a program’s IMS.
A well-structured WBS helps promote accountability by identifying work products that are independent of one another. It also provides the framework for developing a schedule plan that can easily track technical accomplishments—in terms of resources spent in relation to the plan as well as completion of activities—allowing quick identification of cost and schedule variances.
Figure 5: The WBS and Scheduling
A WBS deconstructs a program’s end product into successively greater levels of detail until the work is subdivided to a level suitable for management control. By breaking work down into smaller elements, management can more easily plan and schedule the program’s activities and assign responsibility for the work. It is also essential for establishing a reliable schedule baseline. Establishing a product-oriented WBS such as the one in figure 5 is a best practice because it allows a program to track cost and schedule by defined deliverables, such as a hardware or software component. This allows program managers to identify more precisely which components are causing cost or schedule overruns and to more effectively mitigate overruns by manipulating their root cause.
The WBS is the basis of the program schedule and defines what is required as a deliverable. Scheduling activities addresses how the program is going to produce and deliver what the WBS describes. When properly planned, the schedule reflects the WBS and therefore defines the activities necessary to produce and deliver the lowest-level deliverable. In essence, the schedule is a set of instructions on how the program intends to execute. Every activity within the schedule should be traceable to an appropriate WBS element, and every WBS element must have at least one associated activity that is necessary to complete that element clearly identified within the schedule. Aligning the schedule to the program WBS will help ensure that the total scope of work is accounted for within the schedule.
In the schedule, the WBS elements are linked to one another through the activities’ logical relationships and they lead to the end product or final delivery. The WBS progressively deconstructs the deliverables of the entire effort through lower-level elements.
It is important that the WBS is comprehensive enough to represent the entire program in detail sufficient to manage the size, complexity, and risk associated with the program. There should be only one WBS for each program, and it should match the WBS used for the cost estimate and schedule so that actual costs can be fed back into the estimate with a correlation between the cost estimate and schedule. A well-developed WBS is essential to the success of all acquisition programs.7
In addition, the WBS must have an associated WBS dictionary that clearly defines the scope of each individual WBS element and, therefore, the scope of related schedule activities. Activities that are not assigned to WBS elements or are assigned to undefined WBS elements reflect unassigned or undefined scope. In this manner, the WBS dictionary helps clarify the boundaries between different WBS elements.
Statement of Work
The statement of work (SOW) defines, either directly or by reference to other documents, performance requirements for a contractor’s effort. The SOW defines the scope of the contract—that is, it specifies the work to be done in developing the goods or services to be provided by a contractor. Activities within a contractor’s project schedule should be directly traceable to the program’s SOW.8
The hierarchical nature of the WBS ensures that the entire SOW accounts for the detailed technical activities and, when completed, facilitates communication between the customer and supplier on cost, schedule, resource requirements, technical information, and the progress of the work.
See the GAO Cost Estimating and Assessment Guide (GAO-09-3SP) for numerous examples of standard work breakdown structures for, among others, surface, sea, and air transportation systems; military systems; communications systems; and systems for construction and utilities. For example, DOD has identified, for each defense system, a standard combination of hardware and software that defines the end product for that system. In its WBS standard, DOD defines and describes the WBS, provides instructions on how to develop one, and defines specific defense items (Department of Defense Standard Practice: Work Breakdown Structures for Defense Materiel Items, MIL-STD-881C (Washington, D.C.: October 3, 2011).↩︎
We use SOW as a generic term to represent a document that sets forth the scope of a contract. A statement of objective (SOO) and performance work statement (PWS) may also be used in the contractual process to establish desired service outcomes and performance standards.↩︎