A later refinement considered the process location as well as its spread and tightened the minimum acceptance criterion so that the process mean was at least four sigma from the nearest engineering requirement. Six Sigma requires that processes operate such that the nearest engineering requirement is at least Six Sigma from the process mean. Six Sigma also applies to attribute data, such as counts of things gone wrong. This is accomplished by converting the Six Sigma requirement to equivalent conformance levels, as illustrated in Figure 3.3. Sigma levels and equivalent conformance rates.
Modern business requires near perfect quality levels.
The difference occurs because Motorola presumes that the process mean can drift 1.5 sigma in either direction. Since control charts will easily detect any process shift of this magnitude in a single sample, the 3.4 PPM represents a very conservative upper bound on the non-conformance rate. See Appendix Table 18. For processes with a series of steps, the overall yield is the product of the yields of the different steps. Note that the overall yield from processes involving a series of steps is always less than the yield of the step with the lowest yield.