Control Chart Rules
Statistical textbooks disagree on what constitutes Control Chart Rule 2 – Zone A: 2 of 3 points above/below 2 sigma. Which one is right?
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“Hi, I’m Jay Arthur, author of “Lean Six Sigma Demystified” and QI Macros [software].
“Got a call from a customer this week. He was all upset because of control chart rule two, which is about having points above two sigma or below two sigma (two out of three).
“Well, guess what? Some people in their books say those two points have to be on the same side of the center line.
