Page 182 - Design for Six Sigma for Service (Six SIGMA Operational Methods)

154 Chapter Seven

highest-order function in its operating environment. Although
objectives or specifications are not in themselves functions, they may
influence the method selected to best achieve the basic function(s) and
satisfy the user’s requirements. Note: The use of objectives or specifi-
cations in the FAST process is optional.
Primary Path Functions Any function on the How or Why logic is a
primary path function. If the function along the Why direction enters
the basic function(s), it is a major primary path; otherwise it will be
identified as an independent (supporting) function and be a minor
critical path. Supporting functions are usually secondary. They exist to
achieve the performance levels specified in the objectives or specifi-
cations of the basic functions or because a particular approach was
chosen to implement the basic function(s). Independent functions
(above the critical path) and activities (below the critical path) are the
result of satisfying the When question.
Dependent Functions Starting with the first function to the right of the
basic function, each successive function is dependent on the one to its
immediate left (higher-order function) for its existence. That dependency
becomes more evident when the How question and direction is
followed.
Independent (or supporting) Function(s) Independent (or sup-
porting) functions do not depend on another function or method
selected to perform that function. Independent functions are located
above the critical path function(s) and are considered secondary
with respect to the scope, nature, and level of the problem, and its
critical path.
Activity The method selected to perform a function (or a group of
functions) is an activity.

7.2.4 Symbols and Graphs Used in FAST Diagram Construction
Why, How, and When

Figure 7.5 show the directions in a FAST diagram. The How and Why
directions are always along the primary path, whether it is a major or minor
primary path. The When direction indicates an independent or supporting
function (up) or an activity (down). We have already discussed the How and
Why directions in Example 7.5. The lower-order function on the How
direction (immediate right) always explains how a particular function can be
accomplished; the higher-order function on the Why direction (immediate
left) always tells the reason why a particular function should be performed.