What is Functional Analysis System Technique (FAST)?
A technique to develop a graphical representation showing the logical relationships between the functions of a project, product, process or service based on the questions “How” and “Why
How To Create a FAST Diagram
Three key questions are addressed in a FAST Diagram:
- How do you achieve this function?
- Why do you do this function?
- When you do this function, what other functions must you do?
The following diagram illustrates how a function is expanded in “How” and “Why” directions in a FAST diagram.
Steps in constructing the FAST Diagram
Start with the Functions as identified using Function Analysis:
- Expand the functions in the “How” and “Why” directions:
- Build along the “How” path by asking ‘how is the function achieved’? Place the answer to the right in terms of an active verb and measurable noun.
- Test the logic in the direction of the “Why” path (right to left) by asking ‘why is this function undertaken?’
- When the logic does not work, identify any missing or redundant functions or adjust the order.
- To identify functions that happen at the same time, ask “when this function is done, what else is done or caused by the function?”
- The higher order functions (functions towards the left on the FAST Diagram) describe what is being accomplished and lower order functions (functions towards the right on the FAST Diagram) describe how they are being accomplished.
- “When” does not refer to time as measured by a clock, but functions that occur together with or as a result of each other.
