Achieving Control

The problem which faced Shewhart was how to distinguish between controlled and uncontrolled variation for any given process, and thereby detect the presence of assignable causes. The solution which he devised was the control chart. This simple, elegant and incredibly robust tool provides an eloquent voice for any process, whether it is in manufacturing, or in a service industry; on the shop-floor or at management level.

A control chart is essentially a run chart with lines drawn on to show the process average and the upper and lower control limits. A point which falls beyond either of the control limits is a signal that an assignable cause is present.

There are other indicators of uncontrolled variation, which can be determined by studying the running record. Patterns of variation which do not seem, empirically, to be random are indications of the presence of an assignable cause. We shall look at these later on.