<P> A work breakdown structure permits 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 . </P> <P> 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 . The WBS may be displayed horizontally in outline form, or vertically as a tree structure (like an organization chart). </P> <P> The development of the WBS normally occurs at the start of a project and precedes detailed project and task planning . </P> <P> The concept of work breakdown structure 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 . </P>

The lowest level in work breakdown structure contains