What does the 1.5 shift mean?
What does the "1.5
σ
shift" mean?
Without getting into a statistical and lengthy discussion on what the famous "shift" is, let us say that all processes produce variations over time. In the six sigma methodology (at least in the electronics industry), it was empirically
validated
that the shift of the distribution was about 1.5
σ
. This does not mean that with all processes, and in all industries, this shift is always within this ±1.5
σ
. It does vary. For example, in the automotive industry we know, at least since 1980, that the shift is ±1
σ
and not ±1.5
σ
. Convention now has it that everyone
follows
the 1.5
σ
. One may simplify the interpretation of the shift as a
drift
of the process in the long
term
.
What is the difference between 3
σ
and 6
σ
?
Most companies have been following a standard of performance for ±3
σ
. There is nothing wrong with ±3
σ
for certain products and certain industries. However, there is a tremendous difference between ±3
σ
and ±6
σ
. For example, a ±3
σ
capability accounts for 93.32 percent long-
term
yield (this is the historical standard for most organizations). By comparison, a ±4
σ
accounts for 99.38 percent long-term yield (this is a standard that some organizations
operate
currently). If an organization wants to account for 99.99966 percent long-term yield, then the move to ±6
σ
is inevitable. When the ±6
σ
philosophy is implemented, expect your organization to perform at 3.4 defects per million opportunities. That is truly a
breakthrough
in performance! The percentages presented here have been adjusted for the long-term shift.
What is the DMAIC model?
The DMAIC model is the official methodology for the six sigma problem resolution approach. It stands for define, measure, analyze, improve, control. Fundamentally, the model helps in the following:
What is the DCOV model?
The DCOV model is the official model methodology design for six sigma. It stands for define, characterize, optimize and verify. Fundamentally, the model helps in the following:
-
Defining what the customer needs, wants and expects.
-
Defining the specifications for those needs, wants and expectations.
-
Optimizing the specifications for the specific needs, wants and expectations.
-
Verifying that the needs, wants and expectations are indeed what the customer wanted.
Chapter 5 is devoted to looking at this model in depth.
Is there a real difference between the DMAIC and DCOV models?
Yes, the DMAIC model focuses on appraising quality—it identifies and then
tries
to "fix" the problem. One may say it is a formal approach to solving problems when they occur. On the other hand, the DCOV model is a proactive approach trying to prevent problems from happening. The DCOV model would give a better return on investment and better customer satisfaction.
What does the
Y
=
f
(
x
) expression mean?
In simple mathematical terms, this means that the
Y
is a function of
x.
In plain language, it means that changes in the
x
(i.e., inputs and processes) will determine how the
Y
(i.e., the output) will
turn
out. In the six sigma methodology the
Y
may mean profits, customer satisfaction, strategic goal, efficiency and so on. On the other hand, the x may mean actions that achieve the strategic goals, influences on customer satisfaction, process
variables
and so on. Another way of thinking about this equation is to think of the
Y
as the dependent variable and the
x
as the independent variable.