Figure 7.27 gives the answers to two questions: “Is the process in control?” and “Is the process capable?” (in the sense of the previous paragraph). Each of the four cells in the figure contains some recommended courses of action that depend on the answers to these two
questions. The box in the northwest corner is the ideal state: the process is in statistical control and exhibits adequate capability for present business objectives. In this case, SPC methods are valuable for process monitoring and for warning against the occurrence of any new assignable causes that could cause slippage in performance. The northeast corner implies that the process exhibits statistical control but has poor capability. Perhaps the PCR is lower than the value required by the customer, or there is sufficient variability remaining to result in excessive scrap or rework. In this case, SPC methods may be useful for process diagnosis and improvement, primarily through the recognition of patterns on the control chart, but the control charts will not produce very many out-of-control signals. It will usually be necessary
to intervene actively in the process to improve it. Experimental design methods are
helpful in this regard [see Montgomery (2005)]. Usually, it is also helpful to reconsider thespecifications: They may have been set at levels tighter than necessary to achieve function or
performance from the part.