The aim of this paper is to present the Genichi Taguchi Metric (Loss Function) as compared to the Six Sigma Metric (six sigma = 3.4 defects per million, five sigma = 233 defects per million, four sigma = 6, 210 defects per million, and so on).
On comparing the two metrics it seems that from the point of view of Society (customers, partners, stockholders, suppliers, employees, etc.) a centred process working with four sigma may very well be preferred to an out of the target (m + 1.5 sigma) process working with six sigma, even if the latter produces 3.4 defective per million and the former 32 defective per million (almost 10 tenfold).
The six sigma assumption of a 1.5 s shift in the mean of the process is investigated through the reasons given in the literature and the statement that says: “Since Control Charts will easily detect any process shift of this size (1.5 s ) in a single sample, the 3.4 PPM represents a very conservative upper bound on the non-conformance rate” is critically evaluated.
The question remains in the air as to whether the effort of going from 4s to 6s, under these circumstances, is worth the cost that encompasses.
