Shewhart also observed that it was possible for a process to display uncontrolled variation. He showed that this type of variation could be traced to certain factors which were not fundamental to the process. For this reason, he attributed such variation to Assignable Causes (i.e. causes which could be traced and dealt with appropriately).

A process which displays uncontrolled variation is neither stable nor consistent over time.

The process represented by this diagram is quite different from that on the previous page. At each hour, the distribution of the measurements is different, and the process average is never the same and never on target. It is impossible to tell from one hour to the next whether the process will be able to meet the specifications, and if not, what proportion of the output will be non-conforming.

By correctly applying the methods of SPC, it is possible to bring the process into a state of control, and thereby reduce the overall variation.