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 material defects, equipment malfunction or wear, deviations in operating procedures, human error, or environmental changes such as temperature or humidity. The goal is to locate the specific factor, implement corrective action, and restore the process to statistical control.

If an assignable cause is found and eliminated, process performance often improves and variation reverts toward the baseline level associated with the remaining common causes. If no assignable cause can be identified, observed variation is attributed to common causes, suggesting a process redesign or improvement to raise capability.

Root-cause analysis tools—such as the 5 Whys, Ishikawa (fishbone) diagrams, and Pareto analysis—are commonly used to identify assignable causes. The concept originates from Walter A. Shewhart’s work on control charts and is central to quality management, Six Sigma, and process improvement.