Common Cause vs Special Cause Variation

Every process has variation. The skill in interpreting an SPC chart is knowing which variation is normal and which is telling you something has actually changed.

Common cause variation

Common cause variation is the natural noise of a stable process. Points sit within the control limits and there is no clear pattern. Reacting to common cause variation, often called "tampering", usually makes performance worse, not better.

Special cause variation

Special cause variation is a signal that something has changed. A point outside the control limits, a long run on one side of the mean, or a steady trend are all examples. These deserve investigation, because they have a specific identifiable cause.

How EasySPC helps

EasySPC automatically detects and highlights special cause variation using the standard rules, including points beyond control limits, long runs, and trends. That means you can focus your attention on the signals that matter, instead of squinting at the chart yourself. See the EasySPC plans for the full set of detection rules and Pro features.