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# Six Sigma Capability

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A simplified approach to understanding six sigma capability.
This avoids complexity of capability indeces and is easy to explain to decision-makers.

Published in: Business, Technology
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### Six Sigma Capability

1. 1. Six Sigma Capability (in 3 Power Point Slides!) 1. Here are simplified & easy-to-understand ways to describe defect rates! 2. Let’s fix capability analysis: Not Cp, Cpk, Pp or Ppk! Author: John Haury, Ph.D. [email_address] σ (sigma) & standard deviation (Std) are used interchangeably here. Contact me if you want to discuss short term & long term σ .
2. 2. Confusion: 6 σ versus ±3 σ <ul><li>±3 σ covers ~99.7% of data in a normal distribution! </li></ul><ul><li>±3 σ sounds like 6 σ , 3 left & 3 right of center. 3+3=6. </li></ul><ul><li>±3 σ within specifications seems good until you see the defect rate as 3/1000 or 3000/million. Too expensive! </li></ul><ul><li>± 6 σ within specifications is much closer to ZD. </li></ul><ul><li>± 6 σ within specifications = few defects/billion. </li></ul><ul><li>± 6 σ within specifications = very low costs of failure. </li></ul><ul><li>And “Six Sigma is a business management strategy , that seeks to identify and remove the causes of defects and errors in manufacturing and business processes .” Reference: wikipedia.org/wiki/Six_Sigma. </li></ul>Six Sigma is actually ±6 σ to the nearest specification (minimum).
3. 3. Visualize ± 6 σ (Six Sigma) <ul><li>Plot the histogram (with specification limits) to show the occurrence of defects. </li></ul><ul><li>Overlay best-fit distribution to show probability of defects. </li></ul><ul><li>Show: Defect rate: defects/100, per 1000, per million (ppm) or per billion (ppb). </li></ul><ul><li>Show: Distance to the nearest spec in σ ’s if data is a normal distribution. </li></ul><ul><li>Estimate costs associated w/ defects. </li></ul>Note: Histograms in combination with control charts deliver the essence of process stability and capability analysis!
4. 4. “ See” Capability by Histogram: Defect rate or number of σ to nearest spec -3 -2 -1 0 1 2 3 4 5 6 St Devs  σ Upper Spec.  Histogram & Specs work for any distribution to show defect rate ( w/ or w/out σ ) Cp not needed! After all, it is a unit-less ratio  not easy to explain. Then add Cpk, Pp, Ppk, their confidence intervals & a ±1.5 σ shift? Histograms tell the story! By the way, Cp=Upper minus lower spec. divided by 6 σ (6 times the Std). Cpk is the distance from the mean to the nearest spec (in terms of σ ’s) divided by 3. Conclusion: Defect rates & associated costs!! Now that’s keeping score! Count or estimate the defect rate