Special Cause Variation

Special Cause Variation refers to unexpected, non‑random fluctuations in a process that indicate the presence of external or assignable factors. When a process is in statistical control, most data points cluster around the average, some approach the control limits, and none fall outside these limits. Any deviation from these expected patterns can indicate a special cause.

Control charts are used to identify specific patterns that suggests special causes. These patterns represent statistically unlikely events if the process is stable. These are the types of indications:

• A single plotted point outside the control limits, signals a special cause.
• Seven consecutive points above the average can indicate that the process average has shifted upward due to an external factor.

Recognizing these patterns is essential for identifying the root cause of special causes. All control chart rules focus on detecting patterns that indicate special causes of variation.

To identify these patterns, the chart is divided into these sections above and below the process average:

• 1 Sigma: The zone closest to the average, extending from the mean to ±1 standard deviation (σ).
• 2 Sigma: The zone extends between ±1σ and ±2σ from the average.
• 3 Sigma: The zone extends between ±2σ and ±3σ from the average.