Minitab considers three possibilities for centering values in calculating short-term statistics:
- The target, when one is specified
- The midpoint of the specification range, (USL + LSL)/2, when both specification limits are given but no target value
- The process mean, when only one specification limit is given and no target value is given
The target value does not affect the long-term Z calculations. The long-term Z-values center on the mean of the data.