Work Breakdown Structure in Project Management

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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 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.

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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. History The concept of the Work Breakdown Structure (WBS) developed with the Program Evaluation and Review Technique (PERT) in 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. 1] While the term “work breakdown structure” was not used, this first implementation of PERT did organize the tasks into product-oriented categories.

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. [6] 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, and a set of “common elements” that are applicable to all major systems and subsystems. 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.

WBS design principles The 100% Rule One of the most important Work Breakdown Structure (WBS) design principles is called the 100% Rule.  The Practice Standard for Work Breakdown Structures (Second Edition), published by the Project Management Institute (PMI) defines the 100% Rule as follows: The 100% Rule states that the WBS includes 100% of the work defined by the project scope and captures all deliverables – internal, external, interim – in terms of the work to be completed, including project management. The 100% rule is one of the most important principles guiding the development, decomposition and evaluation of the WBS.

The rule applies at all levels within the hierarchy: the sum of the work at the “child” level must equal 100% of the work represented by the “parent” and the WBS should not include any work that falls outside the actual scope of the project, that is, it cannot include more than 100% of the work… It is important to remember that the 100% rule also applies to the activity level. The work represented by the activities in each work package must add up to 100% of the work necessary to complete the work package. (p. 8) Planned outcomes, not planned actions

If the Work Breakdown Structure (WBS) designer attempts to capture any action-oriented details in the WBS, he/she will likely include either too many actions or too few actions. Too many actions will exceed 100% of the parent’s scope and too few will fall short of 100% of the parent’s scope. The best way to adhere to the 100% Rule is to define WBS elements in terms of outcomes or results. This also ensures that the WBS is not overly prescriptive of methods, allowing for greater ingenuity and creative thinking on the part of the project participants.

For new product development projects, the most common technique to ensure an outcome-oriented WBS is to use a product breakdown structure. Feature-driven software projects may use a similar technique which is to employ a feature breakdown structure. When a project provides professional services, a common technique is to capture all planned deliverables to create a deliverable-oriented WBS.

Work breakdown structures that subdivide work by project phases (e. g. Preliminary Design Phase, Critical Design Phase) must ensure that phases are clearly separated by a deliverable also used in defining Entry and Exit Criteria (e. . an approved Preliminary Design Review document, or an approved Critical Design Review document). Mutually exclusive elements Mutually exclusive: In addition to the 100% Rule, it is important that there is no overlap in scope definition between two elements of a Work Breakdown Structure (WBS). This ambiguity could result in duplicated work or miscommunications about responsibility and authority. Likewise, such overlap is likely to cause confusion regarding project cost accounting. If the WBS element names are ambiguous, a WBS dictionary can help clarify the distinctions between WBS elements.

The WBS Dictionary describes each component of the WBS with milestones, deliverables, activities, scope, and sometimes dates, resources, costs, quality, etc. Level of detail A question to be answered in determining the duration of activities necessary to produce a deliverable defined by the WBS is when to stop dividing work into smaller elements. There are several heuristics or “rules of thumb” used when determining the appropriate duration of an activity or group of activities necessary to produce a specific deliverable defined by the WBS. The first is the “80 hour rule” which means that no single activity or group of activities to produce a single deliverable should be more than 80 person hours long.

The second rule of thumb is that no activity or series of activities should be longer than a single reporting period. Thus if the project team is reporting progress monthly, then no single activity or series of activities should be longer than one month long.  The last heuristic is the “if it makes sense” rule.

Applying this rule of thumb, one can apply “common sense” when creating the duration of a single activity or group of activities necessary to produce a deliverable defined by the WBS. A work package at the activity level is a task that:

  • can be realistically and confidently estimated;
  • makes no sense practically to break down any further;
  • can be completed in accordance with one of the heuristics defined above;
  • produces a deliverable which is measurable;
  • forms a unique package of work which can be outsourced or contracted out. WBS coding scheme.

It is common for Work Breakdown Structure (WBS) elements to be numbered sequentially to reveal the hierarchical structure. For example 1. 3. 2 Rear Wheel identifies this item as a Level 3 WBS element, since there are three numbers separated by a decimal point. A coding scheme also helps WBS elements to be recognized in any written context. Terminal element A terminal element is the lowest element (activity or deliverable) in a work breakdown structure; it is not further subdivided. Terminal elements are the items that are estimated in terms of resource requirements, budget and duration, linked by dependencies and scheduled.

The WBS Construction Technique employing the 100% Rule during WBS construction. There figure (on the right) shows a Work Breakdown Structure (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 (e. g. a spreadsheet) that allows automatic rolling up of point values. Another recommended practice is to discuss the point estimations with project team members.

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Work Breakdown Structure in Project Management. (2018, May 06). Retrieved from

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