The OTR GroupProject teams often reach for a work breakdown structure without asking what the project is meant to produce. A product breakdown structure asks that question first. The two tools sound alike and are often confused, but they answer different questions. This short comparison sets out the difference using the Association for Project Management's (APM) description, and shows how the two fit together.
The product breakdown structure: what are we delivering?
APM describes the product breakdown structure (PBS) as a hierarchical structure with the main output of the project at the top level. Each level beneath shows the components of the level above, down to individual products. APM adds that each product has defined acceptance criteria and quality control methods. In other words, a PBS is a map of what will exist when the project is done, and how anyone will decide that each piece is good enough.
The work breakdown structure: what work creates it?
The work breakdown structure (WBS) shows the work required to produce those products. APM notes that the lowest level of a WBS shows the activities that would be used to create a network diagram for time scheduling. So where the PBS describes things, the WBS describes effort, and it feeds the schedule.
The difference in one line
The PBS shows what is delivered. The WBS shows the work needed to create it. Teams that skip the first and jump straight to the second risk producing a long list of tasks that nobody can tie back to an outcome.
An illustration
Imagine a team launching a small training course (an invented example, for illustration only). Its PBS might have the launched course at the top, with course materials, a booking page and a trainer pack beneath it. Each of those would carry acceptance criteria, such as who reviews the materials and what counts as approved. The WBS would then list the work to produce each item: writing, reviewing, building the page, testing the booking process. Those lowest-level activities are what you would place in a schedule.
A five-step way to use both
- Write the main output at the top. One sentence, in terms of what will exist.
- Break it into components. Keep going until each box is an individual product.
- Add acceptance criteria. For each product, note how it will be judged.
- Build the WBS from the PBS. List the work needed for each product.
- Take the lowest-level activities into your schedule. This is where the network diagram begins.
Common slips
A frequent slip is mixing the two, so that a box reads 'write report' in one place and 'report' in another. A noun belongs in the PBS and a verb belongs in the WBS. Another is stopping the PBS too early, leaving items too large to judge. A third is writing acceptance criteria after the work is done, when they can drift to fit what was produced.
When it helps most
The pairing is most useful at the start of a piece of work, when scope is still fuzzy. Setting out what you are delivering before planning the effort makes disagreements about scope visible early, when they are cheap to settle.
Source and scope
This comparison draws on APM's published description of the PBS and WBS, which cites the APM Body of Knowledge, 7th edition. The APM page sits within members' resources, so read the Body of Knowledge for the full definitions. General information only.