100-Metodos-de-Qualidade-Total

DATA COLLECTION, ANALYSIS AND DISPLAY 209
Cp ppm
1.00 1350
1.33 32
1.67 0.3
2.00 0.001
For attributes data, the capability of the process is often called the first
run capability (FRC) and is calculated as:
FRC = (l -p) X 100%
where p is the percentage of defective items (see Method 87).
Benefits
By establishing the facts about process performance it becomes easier for
technical, marketing and manufacturing departments to talk to each other,
and for companies to talk to their customers and suppliers.
Example
During the manufacture of glass bottles, the internal diameter of the bottle
neck is a critical parameter. The specification of the neck is agreed with
customers as 1.5 cm ± 0.01 cm. The total specification spread is therefore
0.02cm.
Based upon a set of samples of size 5, the mean of the ranges R was
0.0075 cm. Total process spread was calculated as 6 X R/d2 = 0.019. The
capability was therefore 0.02/019 = 1.035. Using the figures given above, it
can be seen that approximately l350 ppm are being produced out of
specification.
Reference
Mal Owens (1989) SPC and Business Improvement. London: IFS Publications.