To provide a timely signal of the presence of instability in a process and thus to indicate when and where to look for causes of instability so that prompt corrective action can be taken. To achieve and maintain stability or statistical control. To pave the way towards the assessment of process capability. Stability does not mean that the process is producing only good products. The inherent variation present in a stable process is called common cause variation.
Statistical techniques can indicate the magnitude of common cause variation.
The causes of instability in a process are called special or assignable causes . Measurement of the product quality characteristics should be made as close to the potential source of problems as possible. Do not wait until the product is completely fabricated or assembled. Remember that control limits reflect the natural tolerance of the process and have nothing to do with specifications. Thus a stable process may be producing items that are consistently out of specifications.