assignablecause
An assignable cause is a factor that produces variation in a process and can be identified, isolated, and corrected. It is also referred to as a special cause variation, in contrast to common cause variation, which is inherent to the process and not attributable to a single identifiable source. In statistical process control, the presence of an assignable cause typically becomes evident when a process output shows non-random behavior, such as a data point outside the control limits or a recognizable pattern within the run of data.
When a control chart indicates out-of-control conditions, investigators search for the assignable cause. Potential sources include
If an assignable cause is found and eliminated, process performance often improves and variation reverts toward
Root-cause analysis tools—such as the 5 Whys, Ishikawa (fishbone) diagrams, and Pareto analysis—are commonly used to