Typically a single sample is taken from each machine and included in the subgroup. What is plotted as common cause variation is really common cause variation plus the slight differences between the process means of the individual machines. Typically, stratification is detected when large numbers of points lie in Zone C of an chart. It looks like the process is in super control when, in fact, the control limits are just calculated too wide. The solution to stratification, obviously, is to chart each machine separately.
Control charts are applicable to one and only one process at a time.
Mixing is similar to stratification, except the output of several parallel machines is mixed and the periodic sample is drawn from the mixture. 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.