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Unit 10: Value Analysis

10.4.1 Creating a FAST Model Notes

The FAST model has a horizontal directional orientation described as the HOW-WHY dimension.
This dimension is described in this manner because HOW and WHY questions are asked to
structure the logic of the system’s functions. Starting with a function, we ask HOW that function
is performed to develop a more specific approach. This line of questioning and thinking is read
from left to right. To abstract the problem to a higher level, we ask WHY is that function
performed. This line of logic is read from right to left.
There is essential logic associated with the FAST HOW-WHY directional orientation. First,
when undertaking any task it is best to start with the goals of the task, then explore methods to
achieve the goals. When addressing any function on the FAST model with the question WHY,
the function to its left expresses the goal of that function. The question HOW, is answered by the
function on the right, and is a method to perform that function being addressed. A systems
diagram starts at the beginning of the system and ends with its goal. A FAST model, reading
from left to right, starts with the goal, and ends at the beginning of the “system” that will
achieve that goal.
Second, changing a function on the HOW-WHY path affects all of the functions to the right of
that function. This is a domino effect that only goes one way, from left to right. Starting with any
place on the FAST model, if a function is changed the goals are still valid (functions to the left),
but the method to accomplish that function, and all other functions on the right, are affected.
Finally, building the model in the HOW direction, or function justification, will focus the team’s
attention on each function element of the model. Whereas, reversing the FAST model and
building it in its system orientation will cause the team to leap over individual functions and
focus on the system, leaving function “gaps” in the system.

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Caution A good rule to remember in constructing a FAST Model is to build in the HOW
direction and test the logic in the WHY direction.
The vertical orientation of the FAST model is described as the WHEN direction. This is not part
of the intuitive logic process, but it supplements intuitive thinking. WHEN is not a time
orientation, but indicates cause and effect.
Scope lines represent the boundaries of the study and are shown as two vertical lines on the
FAST model. The scope lines bound the “scope of the study”, or that aspect of the problem with
which the study team is concerned. The left scope line determines the basic function of the study.
The basic functions will always be the first function to the immediate right of the left scope line.
The right scope line identifies the beginning of the study and separates the input function from
the scope of the study.
The objective or goal of the study is called the “Highest Order Function”, located to the left of the
basic function and outside of the left scope line. Any function to the left of another function is a
“higher order function”. Functions to the right and outside of the right scope line represent the
input side that “turn on” or initiate the subject under study and are known as lowest order
functions. Any function to the right of another function is a “lower order” function and represents
a method selected to carry out the function being addressed.

Those function to the immediate right of the left scope line represent the purpose or mission of
the product or process under study and are called Basic Function. Once determined, the basic
function will not change. If the basic function fails, the product or process will lose its market
value.

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