Is the Process in Statistical Control?

The presence of one or more points above the Upper Control Limit (UCL) or below the Lower Control Limit (LCL) is primary evidence that the process is no longer in statistical control.

Key:

UCL = Upper Control Limit

LCL = Lower Control Limit

= Mean (average of all the values (subgroup averages))

Such plots are very rarely due to common causes of variation, therefore this evidence suggests the existence of a special cause affecting the process.

A plot outside the control limits is a signal for immediate investigation to verify and identify the special cause.

A plot above the UCL could indicate that:

• the sample has been wrongly calculated or the plot misplaced
• the level of variability has increased
• a different measurement method has been used
• a tool has become worn or damaged
• process shift has occurred

A plot below the LCL could indicate that:

• incorrect calculation or plotting
• operator error
• reduced distributution spread
• process shift