As someone who is about to get involved in the area of industrial statistical quality control, I have been reading up on a lot of related material. Clearly 6-Sigma is one such area. On looking at the General Electric website (who seemed to have started the whole thing off), they stated that 6-Sigma is so called because their stated tolerance level of 3.4 defects or less million parts equates to 6 standard deviations under a normal distribution. Idly, I decided to verify this by looking up the one tailed probability of the normal distribution in EXCEL and STATISTICA. But I found that the stated probability level is in fact 4.5 standard deviations NOT 6 standard deviations!
What have I done wrong?
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