Too close to the average
Notice that nearly all the points lie close to the average. This pattern could be caused by a number of circumstances, including:
- Edited data
- Reduced variability without recalculation of control limits
When this pattern occurs, try to establish why. Is this apparent improvement genuine? Can the improvement be maintained? If the improvement can be maintained, then the control limits need to be recalculated. Although the data looks more stable than normal, this condition is referred to statistically as “unstable” .