In the realm of process improvement and quality management, the Control Phase represents the critical final stage of the DMAIC (Define, Measure, Analyze, Improve, Control) methodology. This phase ensures that improvements achieved during the previous stages are sustained over time, and process stability indicators serve as the compass guiding organizations toward consistent, predictable outcomes. Understanding these indicators is essential for anyone seeking to maintain operational excellence and prevent the erosion of hard-won gains.
What Is the Control Phase?
The Control Phase is the culminating stage in the Lean Six Sigma DMAIC framework, where organizations implement measures to sustain improvements and monitor process performance continuously. After investing considerable resources in defining problems, measuring current performance, analyzing root causes, and implementing improvements, the Control Phase ensures that these enhancements become permanent fixtures in daily operations rather than temporary achievements. You might also enjoy reading about Control Phase for Beginners: Everything You Need to Know About Sustainability in Process Improvement.
Process stability indicators act as diagnostic tools that reveal whether a process is operating within acceptable limits or exhibiting concerning variations that require intervention. These indicators help distinguish between normal process variation (common cause) and abnormal variation (special cause) that demands corrective action. You might also enjoy reading about Early Warning Systems: Detecting Problems Before They Become Defects.
The Importance of Process Stability
Process stability refers to the consistency and predictability of a process over time. A stable process produces outputs that fall within expected ranges, allowing organizations to forecast performance, plan resources effectively, and deliver consistent quality to customers. Without stability, even the most sophisticated process improvements will eventually deteriorate, leading to quality issues, customer dissatisfaction, and increased costs.
Consider a manufacturing facility that produces automotive components. If the diameter of a critical part varies unpredictably from one production run to another, assembly line workers downstream will face constant challenges, warranty claims will increase, and customer confidence will erode. Process stability indicators help identify these variations before they cascade into larger problems.
Key Process Stability Indicators
Control Charts
Control charts represent the foundation of statistical process control and serve as the primary tool for monitoring process stability. These visual representations plot process measurements over time, displaying upper control limits (UCL), lower control limits (LCL), and a center line representing the process mean.
Let us examine a practical example from a call center environment. Suppose a customer service department wants to monitor average call handling time to ensure consistent service delivery. Over 25 consecutive days, they collect the following sample data (in minutes):
Day 1: 8.2, Day 2: 8.5, Day 3: 7.9, Day 4: 8.1, Day 5: 8.3, Day 6: 8.0, Day 7: 8.4, Day 8: 8.2, Day 9: 8.1, Day 10: 8.6, Day 11: 8.3, Day 12: 8.2, Day 13: 7.8, Day 14: 8.0, Day 15: 8.5, Day 16: 8.1, Day 17: 8.3, Day 18: 8.4, Day 19: 8.2, Day 20: 8.0, Day 21: 8.1, Day 22: 8.5, Day 23: 8.3, Day 24: 8.2, Day 25: 8.1
The mean of these measurements is 8.2 minutes, with a standard deviation of 0.21 minutes. Using standard control chart formulas, the UCL would be approximately 8.83 minutes and the LCL would be approximately 7.57 minutes. As long as daily measurements fall within these limits and display no unusual patterns, the process is considered statistically stable.
Process Capability Indices
Process capability indices (Cp and Cpk) measure how well a process can meet specified requirements. While control charts assess stability, capability indices evaluate whether a stable process is actually capable of meeting customer specifications.
The Cp index compares the width of the specification range to the width of the process variation. A Cp value of 1.0 indicates that the process spread exactly matches the specification range, while values greater than 1.33 are generally considered acceptable in most industries.
The Cpk index goes further by considering where the process is centered relative to the specification limits. This provides a more realistic assessment of process performance, particularly when the process mean is not centered between the specification limits.
Continuing with our call center example, suppose management has established that call handling time should be between 6 and 10 minutes to balance efficiency with quality service. With a process mean of 8.2 minutes and standard deviation of 0.21 minutes, the Cpk calculation would reveal whether the process consistently meets these specifications while accounting for the current process centering.
Run Charts
Run charts provide a simplified view of process performance over time without the statistical control limits found in control charts. These tools are particularly useful for identifying trends, shifts, and patterns that might indicate process instability.
In a hospital emergency department, administrators might track patient wait times using a run chart. By plotting daily average wait times over several months, they can quickly spot concerning trends such as gradually increasing wait times that might indicate staffing shortages, process bottlenecks, or seasonal variations requiring attention.
Interpreting Process Stability Signals
Understanding what process stability indicators reveal requires knowledge of specific signal patterns that suggest instability:
Points Beyond Control Limits
Any data point falling outside the upper or lower control limits indicates a special cause variation requiring investigation. In our call center example, if Day 26 showed an average handling time of 9.2 minutes (beyond the UCL of 8.83), this would signal an unusual event such as system problems, inadequate staffing, or exceptionally complex customer issues that day.
Trends and Shifts
Seven or more consecutive points trending upward or downward suggest a systematic change in the process, even if all points remain within control limits. Similarly, eight or more consecutive points on one side of the center line indicate a process shift.
If our call center data showed eight consecutive days with handling times below 8.2 minutes, this might indicate that a recent training program successfully improved efficiency, or perhaps that representatives are rushing calls at the expense of quality.
Cyclic Patterns
Repeating patterns at regular intervals often indicate special causes tied to specific conditions such as shift changes, equipment maintenance schedules, or operator rotation. Identifying these patterns allows organizations to address underlying systemic issues.
Implementing Effective Process Controls
Successfully maintaining process stability requires establishing robust control mechanisms:
Standard Operating Procedures
Documented procedures ensure that improved processes are executed consistently across all shifts, operators, and locations. These living documents should be regularly reviewed and updated based on lessons learned and changing conditions.
Training and Competency Verification
Even the best procedures fail without properly trained personnel. Organizations must invest in comprehensive training programs and verify that team members understand and can execute standardized processes correctly.
Response Plans
Predetermined response plans outline specific actions when stability indicators signal problems. These plans specify who is responsible for investigation, what data should be collected, and what corrective actions are authorized at various levels of the organization.
Regular Audits and Reviews
Periodic audits verify that control mechanisms remain effective and that process discipline is maintained. Management reviews ensure that stability indicators receive appropriate attention and that resources are allocated to address emerging issues.
Common Pitfalls in Process Control
Organizations frequently encounter obstacles when implementing process controls. Over-controlling stable processes by reacting to normal variation wastes resources and often increases variation. Conversely, under-controlling by failing to respond to genuine signals allows problems to grow.
Another common mistake involves setting control limits based on specification limits rather than actual process performance. Control limits derived from process data reveal what the process is actually doing, while specification limits reflect what customers require. These are fundamentally different concepts that must not be confused.
The Path Forward
Mastering process stability indicators requires both theoretical understanding and practical experience. These tools provide the foundation for sustaining improvements, reducing variation, and delivering consistent value to customers. Organizations that excel in the Control Phase create competitive advantages through superior quality, lower costs, and enhanced customer satisfaction.
The journey toward process excellence is continuous, and the Control Phase ensures that progress made is never lost to entropy or complacency. By understanding and properly applying process stability indicators, organizations can confidently maintain their improvements while continuously identifying new opportunities for enhancement.
Take the Next Step in Your Process Improvement Journey
Understanding process stability indicators and effectively implementing the Control Phase requires specialized knowledge and practical skills that extend beyond what any single article can provide. Whether you are a quality professional seeking to advance your career, a manager responsible for operational excellence, or an aspiring process improvement specialist, formal training provides the comprehensive foundation you need.
Lean Six Sigma training equips you with proven methodologies, statistical tools, and real-world problem-solving techniques that organizations worldwide rely upon to achieve breakthrough improvements. From Yellow Belt fundamentals through Black Belt mastery, structured certification programs provide progressive skill development supported by experienced instructors and hands-on project work.
Do not let another day pass watching process gains erode or opportunities slip away. Enrol in Lean Six Sigma Training Today and transform your ability to create, measure, and sustain meaningful improvements. Your investment in these skills will pay dividends throughout your career as you lead organizations toward operational excellence and deliver measurable results that impact the bottom line. The processes you stabilize today become the foundation for tomorrow’s innovations.
