Someone asked me why it is six sigma? why not five sigma? Asking positively
I went to a conference in 1994 and Motorola did make a presentation at it about their new “six sigma”. What it came down to was they had implemented SPC in their processes, and the question had come up - how long should we keep continually improving? The answer was determined to be - until the specification limits were six standard deviations from the average line. Sounded like it was more an empirical and experiential thing than a theory (about the same as asking why control limts are at three standard deviations from the average).
I went to a Boeing presentation at the ASQ national in Seattle last year. A Boeing executive said - forget six sigma, in some aircraft applications they were going to nine sigma as the standard.
Because six is better than five.
Six Sigma- 3.4 defects per million opportunities – or 99.9997% error-free. Five Sigma 230 defects per million opportunities – or 99.8% error-free.
Four Sigma 6,200 defects per million opportunities – or 99.4% error-free.
Three Sigma- about 66,810 defects per million- or 93% error-free
Two Sigma- about 308,500 defects per million- or 69% error-free
clients can be assured a high level of satisfaction with six sigma. beyond that i think only the automotive industry and some aviation companies use 12 sigma.
Worked in Motorola for a time, prior to six sigma, we used to call it ‘5 nines’ quality (99.999%). As in 99.99% was good but if we could reach 99.999% quality then we would be ahead of the competitors. The following year six sigma came into vogue.