Use extended limits to account for allowable changes in the process mean and reduce
unnecessary false alarms. For example, when tool wear or material changes shift the
mean but still produce acceptable product, extended control limits reduce signals to
adjust the process.
Note
When you use extended limits, the tests
for special causes that are available for this chart change. Test choices usually
depend on organizational standards.
To calculate extended control limits, select
Extend control limits to account for allowable changes in mean. Then select a calculation method.
Note
The default false alarm probabilities are appropriate for most
analyses. Change these values only to adjust the false alarm rate for extended
limits. Higher values produce charts with narrower control limits.
- Between/Within standard deviation: Use this method to estimate
allowable variation from the between-subgroup standard deviation.
- False alarm probability for within standard deviation (α-within)
- Enter the probability of a false alarm from within-subgroup standard
deviation.
- False alarm probability for between standard deviation (α-between)
- Enter the probability of a false alarm from between-subgroup standard
deviation.
- Min/Max mean method: Use this method to estimate the
allowable variation based on the minimum and maximum subgroup averages.
- False alarm probability for within standard deviation (α-within)
- Enter the probability of a false alarm from within-subgroup standard
deviation.
- Number of subgroup means to calculate min/max location
- Enter the number of subgroup means to use to estimate the minimum and
maximum process locations.
