The lower two boxes in Fig. 7.27 deal with the case of an out-of-control process. The southeast corner presents the case of a process that is out of control and not capable.
(Remember our non technical use of the term capability.) The actions recommended here are identical to those for the box in the northeast corner, except that SPC would be expected to yield fairly rapid results now, because the control charts should be identifying the presence of assignable causes. The other methods of attack will warrant consideration and use in many cases, however. Finally, the southwest corner treats the case of a process that exhibits lack of statistical control but does not produce a meaningful number of defectives because the specifications are very wide. SPC methods should still be used to establish control and reduce variability in this case, for the following reasons:
1. Specifications can change without notice.
2. The customer may require both control and capability.
3. The fact that the process experiences assignable causes implies that unknown forces are at work; these unknown forces could result in poor capability in the near future.