The 1.5 sigma shift is the allowance built into Six Sigma tables: a process with spec limits 6 sigma from the mean in the short term is treated as a 4.5 sigma process over the long run, which is where the famous 3.4 defects per million comes from. A truly centered, stable 6 sigma process would produce about 0.002 ppm. The 1.5 figure is a convention drawn from experience rather than a derived constant, and many statisticians prefer to measure long-term drift directly.
The 3.4 ppm on the slide already bakes in the 1.5 sigma shift. Short-term, that process is a lot cleaner than that.
When a supplier quotes a sigma level, ask whether it is short-term or long-term: the 1.5 sigma gap is the difference between 3.4 ppm and two parts per billion.
Also heard as
- sigma shift
- long-term shift
- Six Sigma shift
Related terms
Sigma level
Process quality expressed as the number of standard deviations that fit between the mean and the nearest spec limit.
DPMO
Defects per million opportunities: defect count scaled by the number of chances for a defect in each unit.
Ppk
Process performance index: a Cpk-style figure computed with the overall standard deviation, capturing long-term variation.
Cpk
Process capability index that accounts for centering: distance from the mean to the nearest spec limit in units of 3 sigma.