Knowing Your PBS From Your PFD

The two product diagrams used in PRINCE2 show the two views of structure and sequence, so they’re very different. Both diagrams are simple in concept. You can’t be asked to draw the diagrams in the exams, because both papers use a multiple-choice question format. However, in the Foundation exam you can be asked general questions about them, and in the Practitioner exam you may be given diagrams and have to answer questions which reflect the project scenario, including finding any errors in the diagrams given and perhaps identifying the correct snippet of diagram to replace the wrong part. Or you may have a diagram with just numbers in the boxes and have to show which products go in which place.

The Product Breakdown Structure (PBS)

The Product Breakdown Structure (PBS) is simply a list. It’s a list of the specialist products but grouped according to their categories and subcategories. The most powerful use of this diagram is when you’re first identifying products. You can just produce a random list of products as they came to mind. It often helps, however, to give some structure to your thinking and to go through the product categories one at a time. If you’re considering a house-building project, for example, you might think about the brickwork products first, then the woodwork products, then the electrical products and then the decorating products. As you think more about the electrical products, you might find it helpful to break them down into two subcategories according to whether they belong to the power circuit or the lighting circuit.

The Product Flow Diagram (PFD)

In contrast, the Product Flow Diagram (PFD) is a sequence diagram. It shows the order in which products will be created during the life of the project. That’s not going to be the same order as the categories in the PBS. Taking the example of the house building again, you might have thought about the brickwork products first, but, looking at the sequence, you wouldn’t develop all brickwork products first in the project. The brickwork for the foundations will indeed be early on in the project, but building the garden wall will come almost last when there’s no more need to bring lorries onto the site.

Fitting the diagrams together

The lowest level of the PBS is the list of products, grouped by the higher level boxes which are merely category labels. The PFD shows those same lowest level boxes, but put in project order. Then the final box on the PFD is the same as the top box of the PBS and represents the whole project. [P2FD Ch14]

The products in the PBS and those in the PFD are exactly the same. It’s just that one diagram shows them grouped in categories and subcategories, while the other shows them in the order in which they’ll actually be created during the project.