Common Causes The two basic categories of variation in output include common causes and assignable causes. Common causes of variation are the purely random, unidentifiable sources of variation that are unavoidable with the current process. A process distribution can be characterized by its location, spread, and shape. Location is measured by the mean of the distribution, while spread is measured by the range or standard deviation. The shape of process distributions can be characterized as either symmetric or skewed. A symmetric distribution has the same number of observations above and below the mean. A skewed distribution has a greater number of observations either above or below the mean. If process variability results solely from common causes of variation, a typical assumption is that the distribution is symmetric, with most observations near the center.