A work breakdown structure or WBS is a tree structure, that permits summing of subordinate costs for tasks, materials, etc., into their successively higher level “parent” tasks, materials, etc. It is a fundamental tool commonly used in project management and systems engineering.
A well-designed WBS forms a product-oriented family tree composed of hardware, software, services, data, and facilities. This means that the WBS is organized around the primary products of the project (or planned outcomes) instead of the work needed to produce the products (planned actions). Since the planned outcomes are the desired ends of the project, they form a relatively stable set of categories in which the costs of the planned actions needed to achieve them can be collected. A well-designed WBS makes it easy to assign each project activity to one and only one terminal element of the WBS.
In addition to its function in cost accounting, the WBS also helps map requirements from one level of system specification to another, for example a requirements cross reference matrix mapping functional requirements to high level or low level design documents.
By June of 1962, DoD, NASA and the aerospace industry published a guidance document for the PERT Cost system which included an extensive description of the WBS approach. This guide was endorsed by the Secretary of Defense for adoption by all services. In 1968, the DoD issued "Work Breakdown Structures for Defense Materiel Items" (MIL-STD-881), a military standard mandating the use of work breakdown structures across the DoD. This standard established top-level templates for common defense materiel items along with associated descriptions (WBS dictionary) for their elements.
The document has been revised several times, most recently in 2005. The current version of this guidance can be found in "Work Breakdown Structures for Defense Materiel Items" (MIL-HDBK-881A).
It includes guidance for preparing work breakdown structures, templates for the top three levels of typical systems (Appendices A through H), and a set of "common elements" that are applicable to all major systems and subsystems (Appendix I)
Defense Material Item categories from MIL-HDBK-881A:
The common Elements identified in MIL-HDBK-881A, Appendix I are: Integration, assembly, test, and checkout; Systems engineering; Program management; Training; Data; System test and evaluation; Peculiar support equipment; Common support equipment; Operational and site activation; Industrial facilities; and Initial spares and repair parts
In 1987, the Project Management Institute (PMI) documented the expansion of these techniques across non-defense organizations. The Project Management Body of Knowledge (PMBOK) Guide provides an overview of the WBS concept, while the "Practice Standard for Work Breakdown Structures" is comparable to the DoD handbook, but is intended for more general application.
A common way of deciding the detailing level is the time between status reports/meetings. If the team reports bi-weekly the largest work package should be 80 hours. Then at reporting time a package is either not started, in progress, finished or late. This way makes it easy catching delays.
A work package is a piece that:
There is a general ‘8/80 Rule’ that states that all and any Work Package should be more than 8 and less than 80 hours in duration.
A terminal element is sometimes called a work package, although the two terms are not synonymous.
Figure 1 shows a WBS construction technique that demonstrates the 100% Rule quantitatively. At the beginning of the design process, the project manager has assigned 100 points to the total scope of this project, which is designing and building a custom bicycle. At WBS Level 2, the 100 total points are subdivided into seven comprehensive elements. The number of points allocated to each is a judgment based on the relative effort involved; it is NOT an estimate of duration. The three largest elements of WBS Level 2 are further subdivided at Level 3, and so forth. The largest terminal elements at Level 3 represent only 17% of the total scope of work. These larger elements may be further subdivided using the progressive elaboration technique described above. In this example, the WBS coding scheme includes a trailing "underscore" character ("_") to identify terminal elements. This is a useful coding scheme because planned activities (e.g. "Install inner tube and tire") will be assigned to terminal elements instead of parent elements. Incidentally, this quantitative method is related to the Earned Value Management technique.
It is recommended that WBS design be initiated with interactive software (i.e. a spreadsheet) that allows automatic rolling up of point values. Another recommended practice is to discuss the point estimations with project team members. This collaborative technique builds greater insight into scope definitions, underlying assumptions, and consensus regarding the level of granularity required to manage the project.
A WBS is not a project plan or a project schedule and it is not a chronological listing. It is considered poor practice to construct a project schedule (e.g. using project management software) before designing a proper WBS. This would be similar to scheduling the activities of home construction before completing the house design. Without concentrating on planned outcomes, it is very difficult to follow the 100% Rule at all levels of the WBS hierarchy.
A WBS is not an organizational hierarchy. Some practitioners make the mistake of creating a WBS that shadows the organizational chart. While it is common for responsibility to be assigned to organizational elements, a WBS that shadows the organizational structure is not descriptive of the project scope and is not outcome-oriented. See also: responsibility assignment matrix (also called a Staffing Matrix).
WBS updates, other than progressive elaboration of details, require formal change control. This is another reason why a WBS should be outcome-oriented and not be prescriptive of methods. Methods can, and do, change frequently, but changes in planned outcomes require a higher degree of formality. If outcomes and actions are blended, change control may be too rigid for actions and too informal for outcomes.
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