The plotted points represent the defect rate for each subgroup, :
Data points on a U chart follow the Poisson distribution.
Term | Description |
---|---|
![]() | number of defects for subgroup ![]() |
![]() | size of subgroup ![]() |
The center line represents the process mean, . If you do not specify a historical value, then Minitab uses the mean from your data,
, to estimate
.
Term | Description |
---|---|
![]() | number of defects in subgroup ![]() |
![]() | size of subgroup ![]() |
Each subgroup rate is converted to a z-score (
):
Next, a moving range of length 2 is used to evaluate the variation in the z-scores and calculate Sigma Z ():
where 1.128 is an unbiasing constant.
Term | Description |
---|---|
![]() | defect rate for subgroup ![]() |
![]() | process mean |
![]() | size of subgroup ![]() |
![]() | moving range of length 2 |
The LCL for each subgroup is equal to the greater of the following:
or
Term | Description |
---|---|
![]() | process mean |
![]() | parameter for Test 1. The default is 3. (For more information go to Select tests for special causes for Laney U' Chart.) |
![]() | size of subgroup ![]() |
![]() | see "Sigma Z" |