The WBS is a Scope Breakdown Structure
The work breakdown structure (WBS) is well recognized as a tool for integrating information on project time and cost performance. Its more fundamental role as a framework for configuration control and scope management has not received the same attention. The logical basis, on which the elements of the hierarchical tree structure are subdivided, in fact, reflects a subdivision of scope. The feature which makes every work package or WBS element unique is its distinct deliverable or output. Once a framework is created through partitioning the scope, the structure is used to identify and classify the work effort through which the deliverables will be obtained.
The hierarchical structure also serves as the information framework that guides the detailing of the components and features of the end product. As additional detail is conceived or decided, the WBS is correspondingly expanded to lower structural levels.
Charette and Halverson observe that the WBS, in its early applications, was a functionally oriented tree structure. It tended to reflect organizational units or contractors, distinguished by discipline or trade. Since the early 1970's, emphasis has shifted to rules of subdivision based on scope (product) or subprojects. The very term "work breakdown structure" has become a misnomer. The WBS, in the current state of the art, is a scope breakdown structure. While earlier definitions of WBS, such as that presented by Wideman tend to refer to work effort, activities, or tasks, the current emphasis clearly is first on the subdivision of project scope, and second on the packages of work. Work packages are planned as means to achieve the outputs reflected in the WBS. The following definition, taken from U.S. Military Standard 881A, is typical of the orientation to scope:
"A work breakdown structure is a product oriented family tree composed of hardware, services, and data which result from project engineering efforts during the development and production of a defense materiel item, and which completely defines the project/program. A WBS displays the product(s) to be developed or produced and relates the elements of work to be accomplished to each other and to the end product".
The services described in the definition mean specific, verifiable actions that are required of the project team by the client. The conduct of training sessions is an example of a service.
Stephanou and Obradovitch emphasize strongly that the WBS should be an end item oriented structure that links objectives and that transcribes the objectives in terms of successively smaller subdivisions of the deliverable end result. They caution against patterning the structure along functional organizational lines, since this leads to a loss of control. The primary focus is on outputs, and the designation of responsibility for the end items follows. Similarly, Spirer specifies that to generate a WBS, one subdivides scope and deliverables in successively more detail until one arrives at elements that can be produced as manageable units of work.
The writer has used the WBS as a management tool on a wide diversity of projects. These include facilities construction, land development, satellite communications systems, social services delivery programs and organizations, and information systems development. From experience, the method of subdividing the required results or end conditions into progressively smaller elements of scope has proven to be an indispensable device for coordinating and communicating the objectives of complex projects. This principle of subdivision has been carried out with good results down to the very lowest levels of the structure. Each critical path network activity, task, work package, or work item is defined in relation to a unique, specific deliverable. The rule, simply put, is that each element must have a scope.
The writer's practice is to charge a single organizational unit with responsibility for the output rather than for the work effort, associated with a work package or activity. This approach overcomes the difficulties that arise from attempting to delineate work packages that draw resources only from a single organizational unit. Multidisciplinary efforts are often required to create an output item or result. In such a circumstance, single discipline or single resource work packages diffuse responsibility for results and foil coordination. Without a focus on deliverables, uncontrollable "level of effort" work packages tend to arise and proliferate.
Spirer notes that certain of the project outputs may not be for delivery to the client, but are needed within the project team to support the decision making process or the performance of the work on "real" deliverables. Such "internal" deliverables are tangible outputs nonetheless and do provide the specification of "scope" about which the work content of WBS elements is planned.
52. Charette, W. and Halverson, W.S. Tools of Project Management, in Stuckenbruck, L.C. The Implementation of Project Management The Professional's Handbook Reading, MA: Addison Wesley, 1981, p122
53. Wideman, R.M. The PMBOK Report. PMI Body of Knowledge Standards, Project Management Journal August 1986, XVII, p30
54. United States Department of Defense. Work Breakdown Structures for Defense Materiel Items MIL STD 881A. Washington, D.C.: Department of Defense, 25 April, 1975, p2
55. Ibid, p23
56. Stephanou, M.S., and Obradovitch, M.M. Project Management Systems and Development and Productivity. Malibu, CA: Daniel Spencer Publishers, 1985, p78-80
57. Spirer, H.F. The Basic Analytical Principles of Project Management. Project Management Journal. December 1984, XV, p78