Common Causes of Variation bring about random variation which is both predictable and characteristic of the process. These will always be present in the process, affecting all its outputs, as long as the process exists in its current form.
Some examples of common causes are:
Because these causes arise from the way the process itself operates, reducing the variation they cause can only be achieved by examining and improving the process as a whole.
Dr. Joseph Juran, one of the pioneers of SPC, has stated recently that typically some 94% of the variation observed within a process can be attributed to these common causes.