Use in the planning process
Both techniques bring different benefits to the planning process and should ideally be used in conjunction to ensure the project is thoroughly planned out.
Together they support a "Design Backwards" approach to the planning process that enables the project manager to clearly:
- Define the outcome
- What is it?
- How will you know that you have achieved it?
- Define what is needed to fulfil the outcome
- Define how what is needed can be produced
- Understand when this can be achieved
- Understand who will be needed to achieve it
You should produce the PBS first so that the project outcome is clearly understood and can be agreed with the sponsor. The PBS clarifies what is to be built or indeed imported from elsewhere. External products represent sub-products sourced externally to the project, not built - for example pre-fabricated components or Commercial Off-the Shelf Software. It is therefore a useful source of risks and external dependencies for the project manager.
The Product Log is a useful place to record suppliers of external products and a Product Flow Diagram enables you to identify the order in which products are required. This enables the logic of the plan to be understood at a high level before detailed planning begins.
A WBS can then be built to organise the construction of products as a set of work-packages and associated tasks. Organising the work into work-packages simplifies resource and team planning and provides the basis upon which a realistic plan can be constructed.
Having “thing” and “work” related views of the plan is very useful in completeness validation.
- Do you have products without work-packages? If so you will not be able to deliver the outcome or have products in the PBS that in the cold light of day aren't required.
- Do you work-packages without products? If so you are either doing unnecessary work or have missed something from the PBS.
It is entirely possible (indeed probable) that the PBS and WBS will look quite different. Products can be represented on the project plan as milestones and linked to the outputs of their associated work-packages. Whatever technique is used there needs to be traceability between the two.
If products will only be partially finished at a stage boundary then intermediate products should be created to ensure that quality checks can be carried out. For example, a test plan may be delivered in draft form for an end-stage review with a view to completion in the next stage.
To be truly effective the WBS and resulting plan need to fit neatly with the organisation structure that will be doing the delivery. Work-packages in particular need to be readily assignable to coherent teams or groups within the organisation.