Six Sigma Capability
4
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 protected] σ (sigma) & standard deviation (Std) are used interchangeably here. Contact me if you want to discuss short term & long term σ.
description
A simplified approach to understanding six sigma capability. This avoids complexity of capability indeces and is easy to explain to decision-makers.
Transcript of Six Sigma Capability
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, [email protected]
σ (sigma) & standard deviation (Std) are used interchangeably here. Contact me if you want to discuss short term & long term σ.
Confusion: 6σ versus ±3σ• ±3σ covers ~99.7% of data in a normal distribution!
• ±3σ sounds like 6σ, 3 left & 3 right of center. 3+3=6.
• ±3σ within specifications seems good until you see the defect rate
as 3/1000 or 3000/million. Too expensive!
• ±6σ within specifications is much closer to ZD.
• ±6σ within specifications = few defects/billion.
• ±6σ within specifications = very low costs of failure.
• 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.
Six Sigma is actually ±6σ to the nearest specification (minimum).
Visualize ±6σ (Six Sigma) • Plot the histogram (with specification
limits) to show the occurrence of defects.• Overlay best-fit distribution to show
probability of defects.• Show: Defect rate: defects/100, per 1000,
per million (ppm) or per billion (ppb).• Show: Distance to the nearest spec in σ’s
if data is a normal distribution.• Estimate costs associated w/ defects.
Note: Histograms in combination with control charts deliver the essence of process stability and capability analysis!
“See” Capability by Histogram: Defect rate or number of σ to nearest spec
-3 -2 -1 0 1 2 3 4 5 6St 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 ±1.5σ shift? Histograms tell the story!
By the way, Cp=Upper minus lower spec. divided by 6σ (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 estimatethe defect rate
