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==='Common' and 'special' sources of variation===
{{Main|Common cause and special cause (statistics)}}
Shewhart read the new statistical theories coming out of Britain, especially the work of [[William Sealy Gosset]], [[Karl Pearson]], and [[Ronald Fisher]]. However, he understood that data from physical processes seldom produced a [[normal distribution]] curve (that is, a [[Gaussian distribution]] or '[[Normal distribution|bell curve]]'). He discovered that data from measurements of variation in manufacturing did not always behave the same way as data from measurements of natural phenomena (for example, [[Brownian motion]] of particles). Shewhart concluded that while every process displays variation, some processes display variation that is natural to the process ("''common''" sources of variation); these processes he described as being ''in (statistical) control''. Other processes additionally display variation that is not present in the causal system of the process at all times ("''special''" sources of variation), which Shewhart described as ''not in control''.<ref>{{cite book |title=Why SPC? |agency=British Deming Association |publisher=SPC Press, Inc. |year=1992}}</ref>
===Application to non-manufacturing processes===
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