who is credited for creating a distinction between common and special cases of variation
Common cause and special cause (statistics) Common and special causes are the two distinct origins of variation in a process, as defined in the statistical thinking and methods of Walter A. Shewhart and W. Edwards Deming. Briefly, "common causes", also called natural patterns, are the usual, historical, quantifiable variation in a system, while "special causes" are unusual, not previously observed, non-quantifiable variation.
Separate but equal The doctrine was confirmed in the Plessy v. Ferguson Supreme Court decision of 1896, which allowed state-sponsored segregation. Though segregation laws existed before that case, the decision emboldened segregation states during the Jim Crow era, which had commenced in 1876 and supplanted the Black Codes, which restricted the civil rights and civil liberties of African Americans during the Reconstruction Era. 18 states had segregation laws.