INTRODUCTION TO SOFTWARE DESIGN FOR SIX SIGMA (DFSS) - Software Design for Six-Sigma: A Roadmap for Excellence - page 199

8.A.4 DIDOVM PHASE: DESIGN

The goal of the design phase is twofold:
Select the best design.
Decompose CTQ(s) into actionable, low-level factors—X(s) referred to as CTQ
flow-down.

Decomposition of CTQ(s) helps to identify correlations and aids in the creation
of transfer functions that can be used to model system behavior and can be used in
prediction of output performance. However, transfer functions may not be derivable
at all times. In such cases, it often is very important to identify the critical factors
X(s), the inputs that are constant or fixed and the items that are noise. For instance, in
designing the DVD player, the DivX transfer function gets represented as shown in
Figure 8.A.4 and helps establish the critical X factors to be controlled for achieving
predictability on the Y(s). This is referred to as CTQ flow-down.
Predicting product performance, also known as capability flow-up, on CTQ(s) is
another key aspect in this phase. It often is difficult to predict performance during
the early stages of product development for CTQ(s) in the absence of a clear set of
correlations. In some cases, this may, however, be possible. For example, in the case
of the DVD player, the CTQ startup time ('Y') and each of the X's 1, 2, and 3 that
contribute to it can be quantified as:

Startup time (Y) = drive initialization (X1) + software initialization (X2)

+ diagnostic check time (X3)

The measurable aspect of the startup time makes it a candidate that will be exam-
ined during the Unit-Testing phase. In CTQ flow-down, the average value of Y and
the desired variation we want in the Y's are used to derive the needed value of X's,

Constants or fixed variables

Memory
/Buffer size Concurrency

X's or Controlled factors

Outputs Y

Index Parsing

Media
AV Content

Divx Playability Index

DivX

Divx Certification

H eader Informatio n

Ex ternal Subtitle

Divx Playback time

Noise variables

Unsupported Codec
(SAN3, DM4V et al)

FIGURE 8.A.4 DivX feature transfer function.