Work breakdown structure

A work breakdown structure (WBS) divides the whole scope of a project into smaller, manageable pieces in a hierarchy. At the top is the project; below it are major deliverables or phases; below those, sub-deliverables; and at the lowest level, work packages that can be estimated, scheduled, assigned to a responsible party and controlled. A good WBS is the backbone of planning: schedule activities, cost estimates, budgets, contracts and progress reports all hang from it.

The WBS is deliverable-oriented: it describes what is to be produced, not how. For a highway project, the second level might be earthworks, pavement, structures, drainage, road furniture and utilities; for a building, substructure, superstructure, envelope, services and finishes. The 100 percent rule says that the elements at each level must together cover all of the work of their parent, no more and no less, so nothing is missed and nothing is counted twice. A WBS dictionary describes each element: scope, deliverables, responsible party, acceptance criteria.

Work packages should be small enough to estimate and control, often a few weeks of work or a reasonable fraction of the budget, but not so small that tracking costs more than it is worth. Each WBS element gets a code that links schedule, cost, procurement and documents. Combining the WBS with the organisational breakdown gives a responsibility matrix and control accounts, the points where cost and schedule performance are measured in earned value management.