Special Cause Variation
Special cause variation refers to a change in output caused by specific factors. These factors include environmental factors or faulty input parameters such as defective raw materials. Special variations can be accounted for and removed from the production process but cannot be mitigated against.
Special Cause Variation Details
In the manufacturing industry, companies expect the output of a certain process to remain at its optimum number. However, certain factors, which we call special cause variations, can change this expectation. We can also refer to these variations as an assignable or exceptional variation. Examples of special variations can include operator error or defective raw materials. A natural disaster is also a special cause variation since no one can predict a natural disaster happening in the middle of the production process.
Managers and overseers of the process cannot predict and therefore cannot mitigate the introduction of these variations. Instead, special variation causes, as the name suggests, go beyond the natural processes. However, the production system can fix these errors once it detects them.
Though it may seem that uncontrollable variables may enter into the production process often, it is not the most common reason for a change in production output. American engineer W. Edwards Demings, who introduced the concept of special cause variation, concluded that only 15% of the errors in the production process are attributed to these unpredictable variances.
Example of Special Cause Variation
Special cause variations are unusual variations that are not quantifiable nor observed previously in a manufacturing process. These variations produce a slightly or greatly significant effect on the output of the process. Take the example of a car battery processing plant. The plant has several machines run by different operators.
One evening, one of the operators is not feeling very well and falls asleep at his packaging station. Because he is asleep, he fails to cue his part of the operation to fall into sync with the rest of the operations. In no time at all, a backlog of batteries waiting to be sent into the packaging station starts piling up on the production line. Luckily, the other operators notice the backlog and remedy the problem immediately.
Other special cause variations may be the delay of a shipment meant to arrive at its destination in two days. Due to poor weather conditions and expressways closures, the shipment is delayed for a week.
Special Cause Variation vs. Common Cause Variation
Special and common cause variations are phenomena that affect the production process yet are linearly opposite. While special variations cannot be predicted or anticipated by historical data, common cause variations are the exact opposite.
Any factors that management can predict and measure that affect the production process are common cause variations. W. Edwards Demings coined this term as well. Companies expect them to occur, they are quantifiable, natural, and usual. These variations are also referred to as natural patterns. Demings estimated that about 94% of all production causes are common causes. Examples of common cause variations include temperature, humidity, working conditions, among others.