In recent months, quality managers Marsha Mallow and her sidekick at Natural Butter Company, Maura Thesame, have bumbled their way through chart interpretation. Beginning with their understanding that a point above the upper control limit or below the lower control limit suggests an out-of-control condition, they have moved to analysis of the charts below. With no points above the upper control limits or below the lower control limits, the chart indicated to the two dauntless leaders that the process was in control and perking along just fine.
Indeed, an out-of-control condition can be suggested by other indicators than the single Procrustean technique they were applying. In this case, the first chart indicates 10 X-bars above ; the second chart has 11 ranges below . The software has issued warning statements: for the first chart, "7 above centerline;" for the second chart, "7 below centerline."
A technician asks Marsha and Maura about the rule of 7, wondering why not 6 or 8. Marsha quickly responds, "We use 7 because it is the square root of 49, a significant number."
Is she correct in this answer?
a) Marsha has hit a lucky number, and happens to be correct.
b) In this case, 7 is not magic with respect to her answer, and she is dead wrong with her knee-jerk response.
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