Each plotted point, , represents the mean of the observations for subgroup, .
Notation
Term
Description
xij
jth observation in the ith subgroup
ni
number of observations in subgroup i
Center line
The center line represents the process mean (). If you specify a historical value for an analysis that allows a
historical value, then the analysis uses the historical value. Otherwise, the
analysis uses the average from your data, , which is calculated as follows:
Notation
Term
Description
sum of all individual observations
total number of observations
Control limits
Upper control limit (UCL)
Minitab calculates a control limit for each specification limit that you enter. If you use a Box-Cox transformation, Minitab transforms the specification limits for these calculations.
The upper control limit is calculated as follows:
Lower control limit (LCL)
The lower control limit is calculated as follows:
where kA is calculated as follows:
Notation
Term
Description
U
upper specification limit
L
lower specification limit
σ̂
estimator for within-subgroup variation
n
subgroup size
p
accepted proportion outside a specification limit
PA
probability of intervention when the proportion outside a specification limit exceeds the
accepted proportion
, represents the mean of the observations for subgroup, 
.
). If you specify a historical value for an analysis that allows a
historical value, then the analysis uses the historical value. Otherwise, the
analysis uses the average from your data, 
, which is calculated as follows:
