Chapter 17 – The Management and Control of Quality
17-39 Six-Sigma Interpretation; Spreadsheet Application (20 minutes)
Sigma One-Tailed Two-Tailed Errors (Defects)
Level Area 1
Area Per Million
1 0.158655254 0.317310508 317,310.51
1Excel formula: = 1 − NORMSDIST(n), where n = sigma level (1, 2, …)
The preceding data indicate suggest a common misconception regarding the quality
level assumed under Six Sigma. Only when a defect is defined as any deviation from
the targeted level of the attribute (i.e., only when the “tolerance” is zero) will the
above approach represent the maximum number of defects per million opportunities
for error. Note, for example, that the expected number of errors (defects) under Six
Sigma is approximately 2 per billion (when any deviation from target is considered a
defect).
In actual practice, based on initial experience by Motorola, the application of Six
Sigma allows some variation (drift) around the target value. That is, there is an
assumption that no process can be maintained in perfect control (i.e., no “drift” at
all). Thus, in practice, a drift of 1.5 standard deviations around the target value is
“allowed.” Any deviation beyond this allowable “drift” would be considered a defect or
out-of-control process.
What this means is that a revised formula is needed to calculate the defects per
million as the Six-Sigma methodology is applied in practice. According to Pyxdek
(http://www.qualitydigest.com/may01/html/sixsigma.html) the Excel formula (under
the assumption of an allowable drift of 1.5 sigma) is: 1000000*(1 − NORMSDIST(Z-
1.5)), where 1.5 = allowable drift (in standard deviations) and Z = Sigma level. For Z
= 6.0, the Excel formula returns: 3.398, the defect-per-million figure commonly, but
perhaps mistakenly, reported in the literature. (Also see, J. R. Evans and W. M.
Lindsay, The Management and Control of Quality, 6th ed. (South-Western, 2005),
Chapter 10.)
17-17
Education.