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Six Sigma is Two Things
• A Highly Capable Process– Long Term Statistical Defect Rate Prediction of 3.4
Defects Per Million or Less
• A Structured Process Improvement Approach– DMAIC = Define, Measure, Analyze, Improve,
Control
What is a Six Sigma?
Six Sigma Process Capability
Six Sigma Capability Can be Expressed at Two Levels
1. At the CTQ Level“The new press has Six Sigma capability on the 2.5mm +/- .05mm press dimension.”
2. At the Overall Process Level“The fuel pump assembly line is a Six Sigma process.”- Methods vary for calculating total process capability, like DPMO or RTY
Six Sigma Process Capability
Mean
0 3-3 1 2-1-2σ σ σ σ σ σ
Nearest Specification
Limit
6σ Minimum Distance
Short-Term Measurement
Data on the CTQ, i.e. “press
dimension XYZ”
A Six Sigma Process Has at Least Six Standard Deviations Between the Mean and the Nearest Spec Limit
When dealing with short-term data, this
1.5σ “buffer” is reserved for future
mean-shift.
Six Sigma Accounts for Mean Shift
0 3-3 1 2-1-2σ σ σ σ σ σ
Nearest Specification
Limit
Data at Time of Project Closure
Worst-Case Future Mean Shift to Right
Worst-Case Future Mean Shift to Left
Six Sigma assumes that the mean will shift by as much as 1.5σ in either direction over the long
run
If long term data is being used, then the 6σ
requirement drops to 4.5σ, since the mean
shift is already present in the data
Process Variation Over Time
Time
Upper Spec Limit
Lower Spec Limit
CTQ
Data Sample at a Point in Time = Short Term Data
Multiple Samples Combined Over the Long Run = Long Term Data
Process Variation Over Time
Time
Upper Spec Limit
Lower Spec Limit
CTQ
Data Sample at a Point in Time = Short Term Data
Multiple Samples Combined Over the Long Run = Long Term Data
Mean Shift
Long Term Versus Short Term Data
6σSHORT TERM = 4.5σLONG TERM
• A process that is 6σ capable in the short term is assumed to drift by as much as 1.5σ over the long run, so the capability will deteriorate to 4.5σ – this is where the 3.4 PPM long term defect rate comes from.
• In the vast majority of cases, data collected is assumed to be short term.
• At the end of a Six Sigma project, sufficient time has not passed to consider the data long term.
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