Work Breakdown Structure (WBS)

A Work Breakdown Structure (WBS) breaks the whole of a project's scope into a tree of progressively smaller pieces, from the top level deliverable down to discrete work packages small enough to estimate, schedule and control. It is deliverable-oriented: each node represents a product or outcome, not an activity, and the 100% rule holds that the children of any node sum to all of the work of that node, no more and no less.

On a construction project the WBS typically runs Location -> Sub-Location -> Discipline -> Activity, so labour, materials and progress can be rolled up at any level. Because every cost and hour is booked against a WBS node, the WBS becomes the spine that Earned Value Management hangs off: Planned Value, Earned Value and Actual Cost are all accumulated per node and summed up the tree.

A clean WBS prevents scope gaps and double-counting, makes responsibility unambiguous, and is the single most important prerequisite for trustworthy SPI, CPI and forecast numbers.