SIX SIGMA FUNDAMENTALS - Software Design for Six-Sigma: A Roadmap for Excellence - page 159

LSL USL

-3 σ

+3 σ

-6 -4 -2 0 2 4 6

FIGURE 7.10 Marginally capable pocess.

Condition II: Marginally Capable Process (see Figure 7.10). The process spread
is approximately equal to the specification spread.

6 σ = (USL − LSL)

When a process spread is nearly equal to the specification spread, the process
is capable of meeting the specifications. If we remember that the process center
is likely to shift from one side to the other, then a significant amount of the
output will fall outside of the specification limit and will yield unacceptable
performance.
Condition III: Incapable Process (see Figure 7.11). The process spread is greater
than the specification spread.

6 σ> (USL − LSL)

LSL USL

-2 σ +2 σ

-6 -4 -2 0 2 4 6

FIGURE 7.11 Incapable process.