Understanding ASQ-CQA -Knowing The Ways
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Relationship to measures of process fallout<br />
<strong>The</strong> mapping from process capability indices, such as Cpk, to measures of process fallout is straightforward.<br />
Process fallout quantifies how many defects a process produces and is measured by DPMO or PPM. Process<br />
yield is the complement of process fallout and is approximately equal to the area under the probability density<br />
function, if the process output is approximately normally distributed.<br />
Φ(σ) =<br />
1<br />
2π<br />
σ<br />
e −t2 /2<br />
−σ<br />
dt<br />
In the short term ("short sigma"), the relationships are:<br />
C pk<br />
Sigma level (σ)<br />
Area under the<br />
probability<br />
density curve Φ(σ)<br />
Process yield<br />
Process fallout(in<br />
terms of<br />
DPMO/PPM)<br />
0.33 1 0.6826894921 68.27% 317311<br />
0.67 2 0.9544997361 95.45% 45500<br />
1.00 3 0.9973002039 99.73% 2700<br />
1.33 4 0.9999366575 99.99% 63<br />
1.67 5 0.9999994267 99.9999% 1<br />
2.00 6 0.9999999980 99.9999998% 0.002<br />
Charlie Chong/ Fion Zhang
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