All variation is caused. There are specific reasons why your weight fluctuates every day, why sales go up, and why Maria performs better than Robert. Management must recognize that variations in production or quality within manufacturing or service processes can be viewed as "special cause" variations, which are best removed by team members operating the process and "common cause" variations, which require management action to change some inherent feature of the process. There are four main types of causes.
Common causes are the myriad of ever-present factors (e.g., process inputs or conditions) that contribute in varying degrees to relatively small, apparently random shifts in outcomes day after day, week after week, month after month. The collective effect of all common causes is often referred to as system variation because it defines the amount of variation inherent in the system.
Special causes are factors that sporadically induce variation over and above that inherent in the system. Frequently, special cause variation appears as an extreme point or some specific, identifiable pattern in data. Special causes are often referred to as assignable causes because the variation they produce can be tracked down and assigned to an identifiable source. (In contrast, it is usually difficult, if not impossible, to link common cause variation to any particular source.) Special Cause variation results from events which are occurring outside the process. For example, a relatively major change in temperature or humidity could cause significant variation (points outside control limits) in the process.
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