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