Some students might ask "Why can't I just make it the way I want it?" The answer is that there are hundreds of variables that affect bolt strength. If a factory does the very best job it can of controlling them all, variation will still exist --- the distribution just becomes narrower like those at the left of Ch 2 Figure 3.
When we consider that the bolt specification also controls lots of other properties, like bolt ductility (stretchyness), bolt dimensions, surface finish and chemical composition, we begin to understand how difficult the task of control is. All of these characteristics have targets in the bolt specification. If our sole concern was bolt strength, eliminating variation would not be possible; but, with the addition of many other requirements, the problem is clearly beyond our ability. The best we can do is to create a process that meets all requirements, with a level of variation that we can tolerate.
Simply put, we can't "just have it the way we like it". We are stuck with a reality of describing what we find in life, in factories and in every other situation.
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Paul F. Watson