Functional Flow FFBDs
A functional analysis looks at the ‘functional’ steps that are required to use or interact with a system. We will use the traditional technique Functional Flow-Block Diagrams (FFBD) for our functional analysis. FFBDs are useful to the design process in as much as they describe the intended use of the system. FFBDs can be hierarchical, and an important consideration is to keep the steps in an FFBD at useful task sizes. Some blocks will have sub-functions, which can then be broken down into a sub-level.
Example applications
FFBDs are used in large projects to highlight both the procedure of functions within a system. An Operational FFBD could be constructed from any step-by-step procedure that you can lay your hands on: baking a cake, putting together IKEA furniture, fixing a car, or sewing a superhero outfit from a pattern. A maintenance FFBD can be constructed from any troubleshooting guide: for example, fixing a phone, repairing a tyre, or tuning a piano.
Similar to…
A journey map or use case diagram. There is a big difference, though. A journey map is a discovery tool for empathising with your user, whereas an FFBD is used to specifically demonstrate the intended procedure for a system. For those who are coders, an FFBD follows similar rules to a procedural programming language: one process must be completed before moving to the next process.
Steps
- Create a bullet list of top-level steps from the start to finish of a procedure. Try to group the steps in the list approximately the same size
- Break the top-level list into more detail by filling in all the second- and third-level steps that are required to use the system
- Look for any maintenance steps. These are steps that require a logical ‘check’, where requiring a check means that a maintenance subroutine will be followed
- Map all of the steps out using the diagramming conventions.
Hints
- Listing the steps in a bullet list can save you heaps of time before moving to a diagram (much easier to change and move around).
- Always draw your diagram left-to-right, use orthogonal lines, and partition the levels clearly
- Once you have created an FFBD, do some user testing and see if the user does exactly what you expect using the steps.
Core resources
- Systems Engineering Fundamentals, Department of Defense Chapter 5 and Supplement 5A [PDF, 7 pages]
- Examples of FFBDs from Blanchard, B.S. and W.J. Fabrycky, Systems Engineering and Analysis, Fifth ed. Pearson, New Jersey, 2011. [PDF, 8 pages]