A new look at the WBS

project breakdown structure

November 1991


A NEW LOOK AT THE WBS: Project Breakdown Structure (PBS)

Robert Youker
Consultant, Washington, D.C.


There is common agreement in the profession on what a work breakdown structure is and what it does. The basic Department of Defense definition is:

3.4 Work breakdown structure (WBS). 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 and defines 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 [3].

Atypical sample is shown in Figure 1.

A recent book on project management states:

Breakdown Structure for Large Communications Project

Figure 1. Breakdown Structure for Large Communications Project

Source: Managing High-Technology Programs and Projects, by Russell D. Archibald. Copyright© 1976 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

The WBS partitions the project into manageable elements of work for which costs, budgets, and schedules can more readily be established. When properly prepared and completed, the WBS helps the Project Manager to assign responsibility for the different technical tasks to specific project personnel…(The WBS also) provides an easy to follow numbering system to allow for a hierarchical tracking of progress. .. Formation of the WBS family tree begins by subdividing, or partitioning, the project objective into successively smaller work blocks until the lowest level to be reported on or controlled is reached. This treelike structure breaks down the project work effort into manageable and independent units that will be assigned to the various specialists responsible for their completion, thereby linking in a very logical manner company resources and work to be performed [2].

In practice, however, this seemingly simple partitioning or factoring down process is not so easy. On actual projects the WBS often must be an artistic blend of a breakdown by (1) subsystems or components (product structure), (2) life cycle phases (process structure), and (3) resource units (organizational structure). In addition, the process of preparing a work breakdown structure is an integral part of the overall process of project and objective definition as well as the preparation of a master summary plan and a linear responsibility chart or matrix. This process is usually easier on physical or “hard projects like equipment or construction and more difficult on “soft” projects like computer programming. The term Project Breakdown Structure (PBS) is more descriptive than Work Breakdown Structure and will be used in the rest of this article.


The basic purposes of the PBS are to:

  1. Divide the overall project into manageable pieces;
  2. Divide up responsibility for the various elements and to relate the work to the organizational chart (resources);
  3. Develop better time, cost and resource estimates based on the smaller elements;
  4. Provide a common base or structure for planning, budgeting, scheduling and cost control;
  5. Relate the project work to the company's chart of accounts;
  6. Go from the broad overall objective down to familiar activities performed by various organizational units;
  7. Define work packages and/or work orders; and
  8. Estimate life cycle costs.

There are some additional benefits of a PBS relating to the CPN (critical path network) plan. The WBS helps to plan out the shape of the network to avoid complicated interfaces in the network. It also allows the CPN planning process to be divided up and to be done separately by the various groups responsible for the different elements of the PBS.

Project Breakdown Structure for Petrochemical Plant

Figure 2. Project Breakdown Structure for Petrochemical Plant

Source: Managing High-Technology Programs and Projects, by Russell D. Archibald. Copyright© 1976 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.


Figure 2 is a sample project breakdown structure based almost entirely on the phases of the life cycle of the project. There is no breakdown by the various physical units or subsystems or components of the plant. On the other hand, in Figure 1 there is no element in the PBS for assembly of the various parts and components into subsystems and finally into outputs. Both Figures 1 and 2, however, have avoided the common mistake of forgetting final items such as manuals, training and spare parts. Also, we can only assume in Figure 1 that system/project management includes the initial preparation of the plan that got the whole project started. A list of common problems encountered when preparing a PBS are

  • Skipping the PBS all together and going directly to the CPN,
  • Using only functions, phases or organizational units instead of end items or components (inputs vs. outputs),
  • Forgetting opening and closing phases such as planning and assembly,
  • Not realizing that the PBS has to include all work on the project
  • Forgetting that elements must be mutually exclusive (getting the same items in two different elements)
  • Not integrating the PBS with the company's cost accounting chart of accounts,
  • Using too much or not enough level of detail,
  • No planning levels of indenture and numbering to provide automatic summary plans for computer software, and
  • Not including, or forgetting, “soft” end items such as services, information or software.


The first and most important (and often the only) approach to preparing a PBS is to break the project or system into subsystems or components based on the physical aspects of the project. This is like chapters of a book, fields in an estate or modules in a computer program. It must include all end items of hardware, software, services and information. This is the product structure.

But we have seen that other work on the project must be included also. This could be phases in the life cycle process (process structure) such as planning or assembly or the work of one organizational unit such as legal (organizational structure). Here it is useful to consider the definition of a major element or subproject which is “a series of interrelated activities which are relatively independent from the rest of the project.” For example, on an airplane project the engine could be developed as a separate subproject by a different company once the specifications and interfaces were clearly defined. Legal could either be a subproject by itself or more likely be activities included in various other subprojects. The elements or categories must be mutually exclusive as well as all inclusive The amount of detail (levels of elements) depends on the purposes of the particular PBS. It is also necessary to look at the existing chart of accounts when developing the PBS, especially its numbering system.

The basic thesis of this article is that the job of preparing a PBS is an artistic blending of the three different structures of process, product and organization into a Project Breakdown Structure or Work Breakdown Structure. Certainly the main approach is physical subsystems, but we must also look at the other structures to ensure that all of the work on the project is included. The PBS must include all of the work on the project.


The process of preparing a PBS can be broken down into a series of steps as defined below and in Figure 3.

  1. Project definition. The objectives, nature and scope of the project need to be completely and clearly defined with all of the end items or outputs clearly identified with complete specifications. A hierarchy of objectives is very useful here which shows a complete ends/means chain.
  2. Level of detail. There needs to be some thinking on the various levels of detail of plans that will be required and the number of levels of elements in the PBS.
  3. Process structure. A chart of the phases of the life cycle of the project should be laid out. (Figure 2, for example.)
  4. Organizational structure. The organization chart for the project should include all of the inputs, groups or individuals that will be contributing to the project, including stakeholders in the project environment if necessary. (Project Management is an example of an organizational unit that often becomes an element in the PBS as in Figure 1.)
  5. Product structure. This is a chart of the breakdown of subsystems or components, including hardware, software, services and information and geographical breakdown if applicable.
    Steps in Planning Project Implementation

    Figure 3. Steps in Planning Project Implementation

  6. Organization's chart of accounts. The establishment of the coding system for the PBS should be based on the existing chart of accounts of the organization.
  7. Project breakdown structure. The above items are woven together into a PBS similar to Figure 1.
  8. Master summary plan (MSP). The level 2 or 3 elements of the PBS become the left side of the MSP and the phases of the project (step 3) become the steps from left to right making up the MSP (see Figure 4). This is the summary plan that can then be detailed out in the CPN. Throughout the project the MSP can be used for reporting to top management.
    Example of a Project Master Schedule for an Electronic Switching Project

    Figure 4. Example of a Project Master Schedule for an Electronic Switching Project

    Source: Managing High-Technology Programs and Projects, by Russell D. Archibald. Copyright© 1976 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

  9. Responsibility matrix. It is useful to analyze the relationships between the elements of the PBS and the organization (see Figure 5 ). The elements of the PBS become one side of the responsibility matrix and the organization chart the other side. Codes can be used inside the cells of the matrix to denote different levels of responsibility of the different actors.
    Sample Responsibility Matrix for Portion of a Project and Organization

    Figure 5. Sample Responsibility Matrix for Portion of a Project and Organization

    Source: Managing High-Technology Programs and Projects, by Russell D. Archibald. Copyright© 1976 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

  10. Project chart of accounts. The PBS coding forms the foundation for the project's chart of accounts.
  11. Critical path network (CPN). The initial planning in the above ten steps forms the basis for preparation of a detailed CPN plan. The PBS does not include activities, time estimates, resource use nor logic and is separate from CPN planning, which is the next step after the PBS.
  12. Work order system. The work order system flows from the definition of work packages in the PBS and the organizational responsibilities. A work package is a convenient breakdown assigned to one organizational unit with clear start and finish points, with a budget value, and with a limited time duration. It is a collection of activities for a specific task such as a subcontract for plumbing. This is ideally suited to the issuance of a work order.
  13. Reporting and control systems. At this point it becomes possible to start thinking about the required reporting system to control progress vs. plan.


Preparing a project or work breakdown structure is a vital part of the overall process of project definition and planning of project implementation. It is more than just the usual factoring down of the product structure into physical components. It is also necessary to look at the process structure (life cycle phases) and the organizational structure (inputs) to ensure that all work on the project is included. These time structures must be melded with artistic blending into a useful project breakdown structure and developed further into a master summary plan and other tools.


Thanks to Jim Sauter of IBM Education for his insight on the Process Structure.


1. Archibald, Russell D. 1976. Managing High-Technology Programs and Projects. New York: John Wiley & Sons.

2. Kezsbom, D. S., Schilling, D.L., Edward, K.A. 1989. Dynamic Project Management: A Practical Guide for Managers and Engineers, p. 78. New York John Wiley & Sons.

3. Military Standard. 25 April 1975. Work Breakdown Structures for Defense Materiel Items. MIL-STD-881A.


Continued from page 36

Robert Youker is an independent consultant and trainer in project implementation and an adjunct professor of project management in the Engineering Management School of George Washington University.

From 1975 to 1987, he worked at the World Bank, first as a lecturer at the Economic Development Institute and later as a management specialist in the Institutional Development Division for Africa. Previous experience includes president of Planalog Management Systems, and analyst with the Xerox Coloration and with Checchi and Company. He is a graduate of Colgate University and the Harvard Business School and has taken graduate work at George Washington University.




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