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What is a Six Sigma?
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
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
A Six Sigma Process Has at Least Six Standard Deviations
Between the Mean and the Nearest Spec Limit
Short-Term
Measurement
Data on the CTQ,
i.e. “press
dimension XYZ”
-3 σ
-2 σ
Nearest
Specification
Limit
Mean
6σ
Minimum
Distance
-1σ
0
1σ
2σ
3σ
When dealing with
short-term data, this
1.5σ “buffer” is
reserved for future
mean-shift.
Six Sigma Accounts for Mean Shift
Six Sigma assumes that
the mean will shift by
as much as 1.5σ in
either direction over
the long run
Data at Time of
Project Closure
Worst-Case Future
Mean Shift to Right
Worst-Case Future
Mean Shift to Left
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
-3 σ
-2 σ
-1σ
0
1σ
2σ
3σ
Nearest
Specification
Limit
Process Variation Over Time
Upper Spec Limit
Multiple Samples Combined Over the Long Run = Long Term Data
C
T
Q
Data Sample at a Point in Time = Short Term Data
Lower Spec Limit
Time
Process Variation Over Time
Upper Spec Limit
Multiple Samples Combined Over the Long Run = Long Term Data
Mean Shift
C
T
Q
Data Sample at a Point in Time = Short Term Data
Lower Spec Limit
Time
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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