Similar to stratification, mixing will mask problems in individual machines and will make isolation of the problem difficult. Mixing tends to produce an appearance on the control chart where points tend to lie near the control limits that they really should be. The more dissimilar the machines, the more pronounced this phenomina will be. Frequently mixtures come from processes such as multispindle screw machines, multicavity molds. The solution to mixing, obviously, is to chart each machine or mold separately.
Control charts are applicable to one and only one process at a time.