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.
Sociolinguistics A commonly studied source of variation is regional dialects. Dialectology studies variations in language based primarily on geographic distribution and their associated features. Sociolinguists concerned with grammatical and phonological features that correspond to regional areas are often called dialectologists.