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Industrial Statistics with Minitab by Xavier Tort-Martorell Llabres, Lluis Marco Almagro, Pere Grima Cintas

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14.8 A More Detailed Data Analysis (Capability Sixpack)

Consider again the file VITA_C.MTW and go to:

Stat > Quality Tools > Capability Sixpack > Normal

nc14uf015.eps
nc14uf016.eps
1. Chart representing the data averages in each subgroup. In the example, there is a clear level jump around the sample 22-23, which causes a big discrepancy between the Within and Overall variability measures. The control limits are computed based on the variability within samples (Within).
2. Chart representing the data ranges in each subgroup. In the example, it indicates that the variation within samples remains stable.
3. Chart representing the individual values of the last 25 subgroups. The displayed default value for the number of subgroups is 25, but it can be modified (Option > Number of subgroups display). In the example, it would be more convenient to fix this value to 40 to have a visual picture of all the data shown aligned with their respective averages and ranges.
4. Capability histogram of the data overlaid with the bell-shaped curves that show the short time variability (Within) and the long term variability (Overall); the widest one corresponds to the long-term variation.
5. Capability normal probability plot of the data. It is used to assess the normality assumption.
6. Width of the variability intervals ...

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