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Distribution Centered 

# of parts  +/ 3 Sigma  +/ 4 Sigma  +/ 5 Sigma  +/ 6 Sigma 
10  97.33%  99.94%  99.9994%  99.999998% 
20  92.74  99.87  99.9989  99.999996 
30  92.21  99.81  99.9983  99.999994 
40  89.75  99.75  99.9977  99.999992 
50  87.36  99.68  99.9971  99.999990 
60  85.03  99.62  99.9966  99.999988 
70  82.76  99.56  99.9960  99.999986 
80  80.55  99.49  99.9954  99.999984 
90  78.40  99.94  99.9948  99.999982 


Distribution Shifted One Sigma 

# of parts  +/ 3 Sigma  +/ 4 Sigma  +/ 5 Sigma  +/ 6 Sigma 
10  79.42%  98.66%  99.968%  99.9997% 
20  63.07  97.33  99.937  99.9994 
30  50.09  96.03  99.905  99.9991 
40  39.78  94.74  99.873  99.9989 
50  31.59  93.47  99.842  99.9986 
60  25.09  92.21  99.810  99.9983 
70  19.93  90.98  99.779  99.9980 
80  15.82  89.75  99.747  99.9977 
90  12.57  88.55  99.715  99.9974 
Interpreting the Capability Indices
Compare the abnormal and normal indices. Capability indices are quite sensitive to assumptions of the distribution.
A Capability index is a statistic, subject to statistical error.
Most Engineers or Black Belts consider a capable process to be one that has a Cpk of 1.33 or better, and a process operating between 1.0 and 1.33 is "marginal." Many companies now suggest that their suppliers maintain even higher levels of Cpk.
A Cpk exactly equal to 1.0 would imply that the process variation exactly meets 3 Sigma. A Cpk exactly equal to 1.33 would imply that the process variation exactly meets 4 Sigma. If the process shifted slightly, and the out of control condition was not immediately, if not sooner, detected, then the process would produce scrap. This is the reason for the extra .33. It allows for some small process shifts to occur that could go undetected. The Table located here, provides an indication of the level of improvement effort required in a process to meet these escalating demands, where "PPM Out of Specification" refers to the average defect level measured in parts per million, (PPM).
Using Defects Per Unit To
Benchmark Different Products
Given equivalent design margins and levels of process control, DPU is proportional to parts count.
Therefore, we can benchmark different products if we know (or can estimate), DPU and parts count.
We normalize products with different parts count by calculating PPM/part
PPM/part = DPU  Parts Count To achieve this in six sigma, you can use our free software.
Also see DPO, (defects per opportunity), and DPMO, (defects per million opportunities).

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Sigma SPC / Jim Winings All Rights Reserved


Last Updated: Sunday, 16Apr06 17:15:27 PDT