Special Cause Variation

Special cause variation is variation in a process that is caused by specific factors, and occurs over and above normal process variation (also known as common cause variation). If measurements from a process are charted in a control chart, special cause variation manifests itself in one of several ways: one or more points above the upper control limit or below the lower control limit; runs of seven or more points all on the same side of the center line, indicating a process shift; or other patterns indicative of a nonrandom pattern (such as trends). Processes that display special cause variation are regarded as being unstable and unpredictable.

The successful management of a process involves the identification of special cause variation and the reason(s) for its presence. However, uncovering special causes is the responsibility of the local workforce, because those who have ongoing exposure to the process are in the best position to correctly identify it. This knowledge then allows for the special cause to be potentially eliminated, leading to greater process stability and predictability. Thus management must provide workers with the ability to act on special cause variation.

The random variation in processes that show no special cause variation is called common cause variation. Process improvement usually involves first the identification and elimination of special cause sources of variation, then the reduction of common cause variation.

As an example of special cause variation, suppose that hospital rooms are subject to an inspection according to some schedule. Based on the inspection criteria, consider the proportion of rooms that pass inspection each day. If we examined these data over a period of, say, a month, we might observe that the average proportion is, say, 95%, with some small variation about this average if we look at the day-to-day value. This would represent common cause variation. If we found a day on which only 60% of the rooms pass inspection, this would be very surprising, and in fact we would seek to identify a special reason, or cause, why this occurred on this specific day. A possible explanation would be that a new room inspector was being used who didn't understand the application of the inspection criteria.