Out-of-Control Signals are critical indicators in Statistical Process Control (SPC) that alert practitioners when a process has deviated from its stable, predictable state. In the Control Phase of Lean Six Sigma, these signals help teams identify when special cause variation has entered a process, …Out-of-Control Signals are critical indicators in Statistical Process Control (SPC) that alert practitioners when a process has deviated from its stable, predictable state. In the Control Phase of Lean Six Sigma, these signals help teams identify when special cause variation has entered a process, requiring investigation and corrective action.\n\nThe most common out-of-control signals are based on rules established by Walter Shewhart and later expanded by Western Electric. These rules are applied to control charts to detect non-random patterns:\n\n1. **Point Beyond Control Limits**: Any single data point falling outside the Upper Control Limit (UCL) or Lower Control Limit (LCL), which are typically set at three standard deviations from the mean.\n\n2. **Run Rule (Seven Points)**: Seven or more consecutive points on one side of the centerline, indicating a shift in the process mean.\n\n3. **Trend Rule**: Six or more consecutive points continuously increasing or decreasing, suggesting a gradual drift in the process.\n\n4. **Two of Three Points**: Two out of three consecutive points falling beyond two standard deviations from the centerline on the same side.\n\n5. **Four of Five Points**: Four out of five consecutive points beyond one standard deviation from the centerline on the same side.\n\n6. **Stratification**: Fifteen consecutive points within one standard deviation of the centerline, which may indicate measurement issues or mixed data sources.\n\n7. **Mixture Pattern**: Eight consecutive points on both sides of the centerline with none in Zone C, suggesting multiple process streams.\n\nWhen out-of-control signals occur, Green Belts must investigate root causes, implement corrective actions, and document findings. Understanding these signals ensures sustained process improvements achieved during DMAIC projects remain stable over time, ultimately protecting customer satisfaction and organizational performance.
Out-of-Control Signals: A Complete Guide for Six Sigma Green Belt
Why Out-of-Control Signals Are Important
Out-of-control signals are critical indicators in the Control Phase of Six Sigma projects because they alert process owners to potential problems before they result in defects or significant quality issues. Understanding these signals helps organizations maintain process stability, reduce variation, and ensure consistent product or service quality. For Green Belt practitioners, mastering out-of-control signals is essential for effective Statistical Process Control (SPC) implementation.
What Are Out-of-Control Signals?
Out-of-control signals are patterns or points on a control chart that indicate a process is no longer operating within its natural variation. These signals suggest that special cause variation has entered the process, meaning something has changed that requires investigation and corrective action.
The most commonly used rules for detecting out-of-control conditions are the Western Electric Rules (also known as Nelson Rules). The primary signals include:
1. Single Point Beyond Control Limits Any single data point that falls outside the Upper Control Limit (UCL) or Lower Control Limit (LCL).
2. Run of Seven (Rule of Seven) Seven or more consecutive points on one side of the center line (mean).
3. Trend Seven or more consecutive points continuously increasing or decreasing.
4. Two of Three Points Two out of three consecutive points in Zone A (beyond 2 sigma from the center line) on the same side.
5. Four of Five Points Four out of five consecutive points in Zone B or beyond (beyond 1 sigma from the center line) on the same side.
6. Fifteen Points in Zone C Fifteen consecutive points within Zone C (within 1 sigma of the center line) on either side - indicates stratification.
7. Eight Points Outside Zone C Eight consecutive points on both sides of the center line with none in Zone C - indicates a mixture pattern.
8. Cyclic Pattern A repeating pattern of highs and lows suggesting systematic variation.
How Out-of-Control Signals Work
Control charts are divided into zones based on standard deviations from the mean:
- Zone C: Within 1 standard deviation of the mean - Zone B: Between 1 and 2 standard deviations from the mean - Zone A: Between 2 and 3 standard deviations from the mean
When data points exhibit patterns that are statistically unlikely to occur due to random chance alone, they trigger out-of-control signals. The probability of these patterns occurring naturally in a stable process is very low, making them reliable indicators of process shifts.
How to Respond to Out-of-Control Signals
1. Stop and investigate the process 2. Identify the root cause of the special cause variation 3. Implement corrective actions 4. Document the findings and actions taken 5. Continue monitoring to verify the process returns to statistical control
Exam Tips: Answering Questions on Out-of-Control Signals
Tip 1: Memorize the Key Rules Focus on the most commonly tested rules: points beyond control limits, run of seven, and trending patterns. These appear frequently in certification exams.
Tip 2: Understand Zone Terminology Be comfortable identifying Zones A, B, and C and their relationship to standard deviations. Questions often reference these zones when describing patterns.
Tip 3: Distinguish Special Cause from Common Cause Remember that out-of-control signals indicate special cause variation, which requires investigation. Common cause variation is inherent to the process and falls within control limits.
Tip 4: Count Carefully When analyzing charts in exam questions, count points methodically. The difference between six and seven consecutive points can change your answer entirely.
Tip 5: Know the Appropriate Response When asked what to do after detecting an out-of-control signal, the correct answer typically involves investigating the root cause rather than adjusting control limits or recalculating the mean.
Tip 6: Recognize Pattern Names Be familiar with terms like shift, trend, cycle, stratification, and mixture as they describe different out-of-control conditions.
Tip 7: Practice with Visual Examples Review sample control charts and practice identifying which rules are being violated. Visual recognition skills are essential for exam success.