Quality

What is the difference between Cp and Cpk?

Cp – It indicates how close a process is running towards the specification limits.

Cpk – It indicates how close a process is running towards the target.

Lets take an example of rifle shooting. When the bullets are shot outside the bull’s eye, but at the same point then it is called Cp, where as when the shots are fired exactly at the bull’s eye repeatedly then it is called Cpk. In other words, repeating a mistake correctly is called Cp, and doing the right thing repeatedly is called Cpk.

Interpretations -

1.      When both Cp and Cpk are SAME, then the process is good and centered (When Cp and Cpk = 1, then process is good)

2.       If Cp and Cpk < 1, then defects are common and poor quality

3.       If Cp and Cpk > 1, then defects are rare and good quality

1. Cp = (USL – LSL) /6*stdev
* USL = Upper Specification Limit. (Is a value below which performance of a product or process is acceptable)
* LSL = Lower Specification Limit. (A lower specification limit is a value above which performance of a product or process is acceptable)
* UCL = Upper Control Limit. (Representing a 3 x sigma upwards deviation from the mean value of a variable)
* LCL = Lower Control Limit. (Similar to Upper Control Limit but representing a downwards 3 x sigma deviation from the mean value of a variable)
2. Cpu = (USL – mean) / 3*stdev,
3. Cpl = (Mean – LSL) / 3 * stdev
4. Cpk = (Minimum of Cpu , Cpl)
5. Defects= As observed
6. Units=As observed
7. Opportunities = As observed
8. DPU=Defects*Units
9. DPO= Defects/(Units*Opportunities)
10. DPMO = DPO*1000000
11. Yield = 1-DPO
12. Yield % = Yield *100
13. Long term sigma = NORMSINV(yiled *100%)
14. Short term sigma = 1.5+Log term sigma
15. Fatal accurracy = 100%-(No. of calls failed due to fatal error/Total no. of calls monitored)
16. Non Fatal Error Accurracy = 1-(Non Fatal Defects/Non Fatal Opportunities)