In the following table, n represents the number of observations and g is the number of groups.
|
Formula |
Abbreviation |
Description |
|
XBAR |
The group average. |
|
XBARBAR |
The average of the group average. |
|
R=Xmax - Xmin |
R |
The range of the group. |
|
RBAR |
The average range of all groups. |
|
S |
The standard deviation of the group. |
|
SBAR |
The average of sample standard deviations. |
|
UCLX |
The upper and lower control limits*. |
|
UWLX |
The upper and lower warning limits*. |
|
UCLR |
The upper and lower control limit for range. |
|
UCLs |
The upper and lower control limits for the standard deviation. |
|
Sigma |
The process standard deviation. |
  |
ZU |
The upper and lower CPK limit. |
|
CPK=MIN(ZU,ZL) / 3.0 |
CPK |
The process capability index. |
*NOTE: The block sets the upper (USLX) and lower (LSLX) user specification limits to values greater and less than the upper and lower control limits, respectively, if you do not specify a value.
A2 is the factor for control limits for X. D4 and D3 are factors for control limits for R. C4 is the divisor estimate of the standard deviation. Additional information on the control charts that these values are derived from can be found in the original text: ASTM publication STP-15D, Manual on the Presentation of Data and Control Chart Analysis, 1976; Pages 134-136.
See Also
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