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Wednesday, May 25, 2022

FAST Diagram: The Function Analysis System Technique

Previously, I shared the six step of value engineering  to systematically improve the value of a products. Within the functional analysis phase (step 2) a FAST (Function Analysis Systems Technique) diagram can be used to deepen the teams understanding of all the functions within the scope of study. The technique uses “How-Why” logic to help describe the project in terms that allow for more effective inventive thinking.

The FAST diagramming is a tool that has been a mainstay with Value Analysis and Value Engineering (VAVE). The FAST diagram provides a graphical representation of how functions are linked or work together in a system (product, or process) to deliver the intended goods or services. By focusing on functions, team and individuals can focus on what is truly important and not ne constrained by physical features of products or processes, leading to better definition of the problem and a clearer path to a solution.

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?

To Create a FAST diagram you:

  • The first step is to brainstorm all the functions the product will serve in the eyes of the customer.
  • The function should be defined as broad and generic as possible, such as produce torque, generate light, shape material etc.
  • The basic function is the overall product function. For example, the basic function of a car seat belt is to restrain a person in a car seat.
  • The Secondary functions:- these are essential to the performance of the basic function and they are direct cause to the basic function.
  • Secondary functions can be categorized into 3 types
    • Required
    • Aesthetic
    • Unwanted by product
  • 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.
  • Overall function is placed at the top left of the diagram. The sub-functions resulting from how questioning are listed to the right of the overall functions.
  • This string of sub-functions that are critical to achieving the product function is called Critical Path.
  • Other Functions must be listed at the top of the Fast diagram.
    • All Time Functions are functions that are pervasive to the product. Listed on the right of the diagram.
    • One Time Functions are functions are listed at the centre of the diagram.

Let us take an example look at how a FAST diagram looks like for a product. Then we can understand the steps very easily.


The development of a FAST diagram is a creative thought process which supports communication between team members.

There is no 'correct' FAST diagram but there is a valid method of representing the logic in a diagram. The validity of a FAST model for a given situation is dependent on knowledge and scope of the workshop participants. The FAST diagram aids the team in reaching consensus on their understanding of the project.

The development of a FAST diagram is a creative thought process which supports communication between team members.

The development of a FAST diagram helps teams to:

  • Develop a shared understanding of the project
  • Identify missing functions.
  • Define, simplify and clarify the problem.
  • Organize and understand the relationships between functions.
  • Identify the basic function of the project, process or product.
  • Improve communication and consensus.
  • Stimulate creativity.

FAST Diagrams represent a very powerful fool in analyzing complex systems, and boiling down the descriptions of the functions performed into the essence of the tasks that must be delivered. The requirement that functions be described in a two-word verb-noun format is a double-edged sword:

  • It promotes creative thinking in alternate ways to deliver a function; and
  • Limits it by constraining the perspective to looking at only positively framed functions.

The How/Why logical connection between functions serves as an excellent way to verify the validity of FAST Diagrams, and the focus on functions removes the physicality from the system (product, process, or service), opening the creative vista and removing physical and mental constraints in the process. Functional analysis helps make complex systems more understandable. 

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