Inherent variability

A certain level of variability in any process is unavoidable. We say that this 'acceptable' level of variability is a result of common causes (or random causes) and if it is the only source of variability, the process is said to be in control.

Systematic changes

Our aim is to detect changes to the output that are not the result of common causes. Such systematic changes are said to be the result of special causes (or assignable causes) and could result in...

Systematic changes usually indicate problems with the quality of the output and the process is said to be out of control.

Run chart

A control chart plots measurements describing the process in time order — a type of time series plot. We hope to use it to quickly detect problems and adjust the process to maintain quality. If a single value is recorded at each time, the control chart is called a run chart. (Other control charts plot sample means and ranges.)

The challenge is to detect systematic changes in the control chart (due to special causes) over the background level of variability (due to common causes).

In the example below, the manager has determined that an output measurement should 'almost always' be between 2,000 and 2,080 with variability in this range considered to be due to common causes. Two out of the last four values were above the upper limit, suggesting a problem with the process that should be investigated and fixed.