The variation within the upper and lower control limits ( UCL and LCL ) is considered the natural variation of the process.
2.
At the least, any points above either upper control limits or below the lower control limit are marked and considered a signal of changes in the underlying process that are worth further investigation.
3.
These rules are evaluated for one side of the center line ( one half of the control band ) at a time ( e . g ., first the centerline to the upper control limit, then the centerline to the lower control limit ).
4.
Next, the upper control limit ( UCL ) and lower control limit ( LCL ) for the individual values ( or upper and lower natural process limits ) are calculated by adding or subtracting 2.66 times the average moving range to the process average:
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