Special vs. Common Causation

Special vs. Common Causation

W. Edwards Deming, one of the fathers of manufacturing quality control, explained the difference between special causes and common causes. He was speaking of the causes of defects in manufacturing processes.

He explained that sometimes someone does something obviously wrong, a machine malfunctions or raw material has an obvious flaw. When such an event causes a defect in manufacturing, that defect has a special cause. However, sometimes everyone performs normally, machines function as usual, raw materials meet specs and still a defect happens.

Such defects, according to Deming, have common causes. In other words, the cause of the defect is common to the process. It is built in and does not require outside intervention to make it happen. According to Deming, such defects may not happen frequently, and often may not be accurately assessed or diagnosed.

Think how this dichotomy applies to safety. An accidental workplace injury, just like a manufacturing defect, is an undesired and unplanned outcome of a process. Work processes are designed to produce products or services, not defects or injuries.

The causes of workplace injuries can fall into the same two categories as the causes of manufacturing process defects. A worker can do something wrong, a machine can malfunction or irregular events of other kinds can happen. When these things result in accidental injuries, such injuries could be said to have special cause. However, when organizations investigate accidents and fail to find irregular conditions or behaviors as root causes, they do not always consider the alternatives. It is at this level of investigation that Deming's observations can help improve safety.

Three Practices That Hamper Efforts

There are three heritage practices that hamper the application of common-cause thinking in many safety efforts. Before we look at the solution, consider how one or more of these three might impact your safety thinking and need to be addressed before this common-cause approach can help.

The first is the tendency to manage safety by exception. As long as the lagging indicators are okay, many organizations simply crank the safety activities. Once an accident or serious near-miss occurs, the reactive mechanism fires up.

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EHS Today is an IndustryWeek companion site within Penton's Manufacturing & Supply Chain Group.


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