In Six Sigma, the long-term process is assumed to vary by plus and minus 1.5 standard deviations (sigmas) as compared with the short-term process. Thus, the Six Sigma defect rate of 3.4 DPMO is actually the defect rate for true 4.5 sigma capability. Thus, all capability data and defect rates should be labelled short-term (or “no sigma shift”) or long term (or “with sigma shift”). “
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