Work breakdown structure

A work-breakdown structure (WBS) in project management and systems engineering is a breakdown of a project into smaller components. It is a key project management element that organizes the team's work into manageable sections. The Project Management Body of Knowledge defines the work-breakdown structure 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." A WBS provides the necessary framework for detailed cost estimation and control while providing guidance for schedule development and control.

Overview WBS is a hierarchical and incremental decomposition of the project into deliverables (from major ones such as phases to the smallest ones, sometimes known as work packages). It is a tree structure, which shows a subdivision of effort required to achieve an objective, for example, a program, project, and contract. In a project or contract, the WBS is developed by starting with the end objective and successively subdividing it into manageable components in terms of size, duration, and responsibility (e.g., systems, subsystems, components, tasks, subtasks, and work packages) which include all steps necessary to achieve the objective.

The work breakdown structure provides a common framework for the natural development of the overall planning and control of a contract and is the basis for dividing work into definable increments from which the statement of work can be developed and technical, schedule, cost, and labor hour reporting can be established. A work breakdown structure permits the summing of subordinate costs for tasks, materials, etc., into their successively higher level "parent" tasks, materials, etc. For each element of the work breakdown structure, a description of the task to be performed is generated. This technique (sometimes called a system breakdown structure) is used to define and organize the total scope of a project. 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 cross-reference matrix mapping functional requirements to high level or low-level design documents. The WBS may be displayed horizontally in outline form or vertically as a tree structure (like an organization chart). The development of the WBS normally occurs at the start of a project and precedes detailed project and task planning. Through Progressive elaboration, an iterative process in project management knowledge, the details of project management plan and amount of information will increase, and initial estimates of items such as project scope description, planning, budget, etc. will become more accurate. It also helps the project team to make the project plan with more details.

Types PMI's Practice Standard for Work Breakdown Structures identifies two major types of work breakdown structures.

Deliverable-oriented WBS Deliverable-oriented WBS, also known as Product breakdown structure uses key deliverables to group each work in the project.

Phase-oriented WBS Phase-oriented WBS groups the work under key phases or stages of project lifecycle.

History The concept of work breakdown structure was developed with the Program Evaluation and Review Technique (PERT) by the United States Department of Defense (DoD). PERT was introduced by the U.S. Navy in 1957 to support the development of its Polaris missile program. While the term "work breakdown structure" was not used, this first implementation of PERT did organize the tasks into product-oriented categories. By June 1962, DoD, NASA, and the aerospace industry published a document for the PERT/COST system, which described 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 requiring the use of work breakdown structures across the DoD. The document has been revised several times. As of May 2023, the most recent revision is F, released 13 May 2022. The version history and current revision of the standard are posted on the Defense Logistics Agency (DLA) ASSIST web site.[1] It includes WBS definitions for specific defense materiel commodity systems and addresses WBS elements that are common to all systems. Defense Materiel Item categories from MIL-STD-881F are:

Aircraft Systems Electronic/Generic Systems Missile/Ordnance Systems Strategic Missile Systems Sea Systems Space Systems Ground Vehicle Systems Unmanned Maritime Systems Launch Vehicle Systems Information Systems/Defense Business Systems The common elements identified in MIL-STD-881F, Appendix K are: Integration, assembly, test, and checkout; Systems engineering; Program management; System test and evaluation; Data; Peculiar support equipment; Common support equipment; Operational/Site activation; Contractor Logistics Support; Industrial facilities; Initial spares and repair parts. The standard also includes additional common elements unique to Space Systems, Launch Vehicle Systems, and Strategic Missile Systems. In 1987, the Project Management Institute (PMI) documented expanding 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 standard but is intended for more general application.

Design principles

100% rule An important design principle for work breakdown structures is called the 100% rule. It has been defined as follows:

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