# Applying Design for Six Sigma to Software and Hardware Systems: Requirements Flow-Down

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## Capability Analysis

The next step shown in Figure 10.2 involves preliminary assessment of design capability. The design capability is a predicted capability, as opposed to the measured capability assessed on existing products or processes in the DMAIC process improvement flow. The preliminary assessment is performed in the Design phase of RADIOV (corresponding to the Measure phase of DMADV); if it is inadequate, then later steps (largely in the Optimize phase of RADIOV or Analyze through Design phases of DMADV) will improve the capability and a new assessment of the design capability will presumably be reflected in an improved value for the design capability indices. Later, in the Verify phase, the actual capability will be assessed on prototypes or early production samples, as discussed in Chapter 16.

As a predicted capability, the design capability might be assessed using predictive methods such as Monte Carlo simulation or a method referred to as the propagation of errors or system moments method in some situations and the root sum of squares method in other situations. These predictive engineering methods are discussed in Chapter 14.

There are two key indices used to assess design capability: the Cp (also known as Pp) and the Cpk (also known as Ppk). Equations for these two indices are given here:

Equation 10.1

Equation 10.2

Six Sigma performance is defined as having a Cp greater than or equal to 2 and a Cpk greater than or equal to 1.5. It is possible that the initial design capability assessment will forecast Cp and Cpk values that meet Six Sigma performance expectations at the get-go. If the team has confidence in this initial estimate, the design team can breathe a sigh of relief, celebrate, party, and paint the town red as appropriate to their personalities and local laws and customs. In addition to this emotional reaction, the design team need not expend any further effort on this critical parameter unless something changes that would jeopardize this pleasant state of affairs. Consequently, the detailed flowchart in Figure 10.2 shows that the flow-down for that critical parameter can be considered complete, and the design can move on to the efforts for the next critical parameter.

In those cases in which the initial assessment of the design capability do not provide sufficient confidence that the initial design is capable, the next step would entail flow-down or decomposition, as discussed in the next section.