If a capability index cannot be calculated, Minitab inserts an asterisk (*) next to the index in the capability analysis results. If you see an asterisk in your output, it may be due to one of the following reasons.
- Only one specification limit
- If you enter only an upper or a lower specification limit, but not both, Pp and Cp indices cannot be calculated. Pp and Cp are both calculated using the distance between the lower and upper specification limits. If only one specification limit is entered, that distance does not exist, and Cp and Pp values cannot be reported.
- No target
- If you do not enter a target for the analysis, Cpm cannot be calculated. Cpm is a capability index that measures whether the process meets specification and is on target, by taking into account the data's deviation from the target value. If no target is entered, that deviation does not exist, and Cpm cannot be reported.
- Two boundaries
If you select both specification limits as boundaries, all capability statistics will have asterisks (*) instead of values in the output.
You should only use a boundary when it is not possible for data to exceed that specification limit. For example, if you record downtime for a piece of equipment, a lower specification of zero should be checked as a boundary because negative downtime is not possible.