SN RatioThe concept, "We should act in design step to get stable quality" is called robust design. To do robust design, we have to find important factors and control them.
*** "Robust design" is image of ideal design. This is a target of quality engineering , but there is not perfect method.
Quality engineering for design wants to improve the unevenness of quality. Not average. Average is the second target. The cost caused by unevenness is large.
Because of this concept, quality engineering succeeded.
"Target is robust. (concept)"
--> "We want to control unevenness."
--> "Index is
SN ratio (mathematical tool)"
Of course, using Standard Deviation as an index is not bad.
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