Work breakdown structure

What it means

A WBS, work breakdown structure, is a hierarchical decomposition of a project into smaller, manageable parts down to individual work packages. Each package has a clear deliverable and an owner, and can be planned, costed and tracked separately.

At the top level is the whole project, below it the main parts or phases, then smaller components and, at the lowest level, specific work packages. For example, when implementing a new accounting system, the main parts might be supplier selection, data migration, configuration, training and go-live.

The WBS is a fundamental tool of project management. It ensures that nothing essential is forgotten and creates a common language for the team, the client and suppliers. The rule is that the WBS contains the entire project scope, nothing more and nothing less.

How it works in practice

The WBS is the starting point for estimating cost and time. When work packages are small enough, they can be estimated more reliably and their sum gives the project budget. The schedule, often shown as a Gantt chart, is then built on the WBS.

The schedule reveals the critical path, the sequence of activities whose delay would push back the whole project. Work packages are often linked to milestones marking key points, such as completing data migration or signing off testing.

When tracking progress, the WBS is used to compare plan with actuals, for example with earned value management. For each package it shows whether it is done, what it cost and whether it is on plan, allowing problems to be spotted early.

What to watch out for

A WBS that is too coarse does not allow reliable estimates and control, while one that is too detailed creates needless administration. A sensible level is work packages lasting from a few days to a few weeks, each with a clear and verifiable deliverable.

If the project scope changes during execution, the WBS, budget and schedule must be updated too. Unrecorded scope creep is a common cause of budget and deadline overruns, as the project management triangle illustrates.