Special Cause Variation

Common- and special-causes are the two distinct origins of variation, in a process that features in the statistical thinking and methods of Walter A. Shewhart and W. Edwards Deming. However, it can be argued that they were recognised and discussed as early as 1703 by Gottfried Leibniz and are particularly important in the thinking of economists Frank Knight, John Maynard Keynes and G. L. S. Shackle. Several alternative names have been used over the years.
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a variation in output for a process because something about the process changed, such as, wear-out a setting was changed, something broke in the process equipment, parts were changed, etc. Special cause variation can be desirable or undesirable, for example, if a part on a machine is fixed and the variability in machine output is suddenly decreased, that would be a desirable special cause, alternately, if a part on a machine breaks increasing variability that would be undesirable.