Between/within capability analysis is based on the following four standard deviations:
Within-subgroup standard deviation
σwithin is an estimate of the variation within subgroups (for example, one shift, one operator, or one material batch). Minitab estimates σwithin using one of the following methods:
Between-subgroup standard deviation
σBetween is an estimate of the variation between subgroups (for example, subgroups collected at set intervals, batches, or by different operators).
is estimated using one of the following methods:
Between/within standard deviation
|σ2Between||Variance between subgroups|
|σ2within||Variance within subgroups|
Overall standard deviation
By default, Minitab does not use the unbiasing constant when estimating σoverall. σoverall is estimated by S. If you want to estimate overall standard deviation using the unbiasing constant, you can change this option on the Estimate subdialog box when you perform the capability analysis. If you always want Minitab to use the unbiasing constant by default, choose and select the appropriate options.
|Xij||The jth observation in the ith subgroup|
|X̅||Process mean |
|ni||Number of observations in the ith subgroup |
|C4 (N)||Unbiasing constant (as defined for the pooled standard deviation)|
|N (or Σ ni)||Total number of observations |