Tool 3 - Control chart

A control chart is a graph that represents the behaviour of a process over time. Also called a Shewhart chart, this statistical process control tool is used to indicate whether a process is functioning in control. The Control chart can provide critical information on operational parameters of a process, and so is sometimes called the process-behaviour chart.

The control chart plots measurements of a specific process quality characteristic (Y-axis) against time (X-axis). Predetermined measurement values are used to set the Upper (UCL) and Lower control limits (LCL) for the process. If all measurements lie within these limits it indicates the process is in control. The Central control limit (CCL) is calculated as the mean of the values, and is invaluable for further statistical analysis.

The "Rule of 7" states that seven consecutive results on one side of the mean could indicate a problem. Thus even when a control chart appears to be in control it could indicate a skew problem. 

The ASQ, highlights additional out-of-control indicators:

1) A single point outside the control limits

2) Two out of three successive points on same side of centerline and more than 2σ from it

3) Four out of five successive points on same side of centerline and more than 1σ from it

4) Eight points in a row on the same side of the centerline (Similar to the Rule of 7)

5) Also, obvious consistent or persistent patterns that could suggest something unusual about the data, process or the data gathering technique

As the name suggests, the process-behaviour chart is most efficient when results need to be predicted for an ongoing process. If the process is determined to be in control the control chart can be used to find and correct problems as they occur.

For process improvement the Shewhart chart can provide inputs to decide which specific problem points need to be addressed, or also if the complete process needs to be overhauled.

NB: A run chart is a "naked" form of control chart without the concept of control limits. The run chart simply displays process measurements over time.