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.