Sigma level converts a defect rate into a count of standard deviations, using the Six Sigma convention that adds a 1.5 sigma allowance for long-term drift. On that scale 3 sigma corresponds to about 66,800 defects per million opportunities, 4 sigma to 6,210, 5 sigma to 233 and 6 sigma to 3.4. It is popular in management reporting because it puts very different processes, from machining to invoicing, on one scale.
Billing errors went from 3.8 to 4.6 sigma level after the form change, roughly 11,000 down to 1,000 DPMO.
Also heard as
- process sigma
- sigma
- sigma score
Related terms
1.5 sigma shift
Six Sigma convention that assumes a process mean wanders by 1.5 standard deviations over the long term.
DPMO
Defects per million opportunities: defect count scaled by the number of chances for a defect in each unit.
Cpk
Process capability index that accounts for centering: distance from the mean to the nearest spec limit in units of 3 sigma.
Process capability
How well a stable process's natural spread fits inside the specification limits, usually expressed as an index.