Last edited 17 Aug 2019

Work breakdown structure

Work breakdown structures are used mainly in project management and systems engineering. They were first developed by the US Defense department in 1957 as part of the development of the Polaris missile programme and further developed by NASA in 1962.

A work breakdown structure (WBS) is a system of breaking down a project or programme into manageable tasks, phases, deliverables or work packages (subdivision of effort). However, it is not a recipe for how a project should be constructed nor is it just a list of tasks, a schedule or an organisation chart - although it provides a basis from which a schedule or task list can be created. The benefit of a WBS is that it allows detailed thinking at the micro level without losing sight of the macro picture.

An example is the building of a house which represents the top-level deliverable. Starting with this end objective, WBS determines a hierarchy by subdividing the project into manageable components (or sub-deliverables, eg systems, subsystems, components, tasks and subtasks), thereby defining all the necessary steps to achieve the end goal (ie the finished building) in increasing levels of detail.

For example, this may mean:

Level 1 Top-level deliverable or project (the finished building).
Level 2: Sub-dividing the project into main components, eg substructure and superstructure.
Level 3: Breaking down each component into smaller components (eg substructure > excavation).
Level 4: breaking down each component into more components. (eg excavation > place reinforcement)
Level n: Final breakdown into smallest component (eg place reinforcement > pour concrete)

The process is broken down until a point is reached at which sufficient detail exists to allow precise planning and management. This lowest level is known as Level n (see above). At this level, the work is assigned to a specific department or unit to undertake the work. Furthermore, each of the Level n components can have its own cost account and allocated its own budget to produce the deliverable. In this way, integrating cost with the WBS allows the tracking of financial performance in addition to project performance.

There should be no scope overlap between the different elements of a WBS. If this was the case, there could be ambiguity and possible duplication of work, as well as cost-accounting confusion.

In terms of representation, a WBS can be shown vertically as a tree or horizontally in outline form.

The 100 per cent rule is a central tenet of WBS that guides its development, composition and evaluation. It holds that a WBS will contain 100% of the work defined by the project scope and will account for all deliverables (whether internal, external or interim work to be completed). The WBS can only include 100% of the work of the project and should not include any work that falls outside the project scope. All the work in each work package must add up to 100% of the work necessary to complete that work package and all work packages must add up to 100% of the top-level deliverable. This is analogous to a pie chart in which the total area of all segments (irrespective of how many segments there are) must be equal to the total area of the circle.