Page 545 - Design for Six Sigma a Roadmap for Product Development

504 Chapter Fourteen

A 0

Nominal-the-Best
Y
T – D 0 T T + D 0

A 0
Larger-the-Better
Smaller-the-Better
A 0
Y Y
D 0 D 0
Figure 14.4 Quality loss functions.

So we can see that there are two components in quality loss:

1. Off-target. This component corresponds to the deviation of mean
performance level from the target value and can be represented by
2
k( y T) .
2. Variation. This corresponds to the variation of performance level,
represented by k Var(Y).

If we want to reduce the quality loss, we need to reduce both off-target
and variation components.

14.2.2 Sources of variation
From Eq. (14.8)

2 2 2 2
L kE(Y T) k( y T) k Var(Y) k( y T) k y

It is clear that in order to reduce the quality loss, we should reduce
2
both off-target k( y T) and variation components, as illustrated in
Figs. 14.5 and 14.6.