The Control Phase represents the final and arguably most critical stage of the DMAIC (Define, Measure, Analyze, Improve, Control) methodology in Lean Six Sigma. After investing significant time and resources into improving processes, the sustainability of these improvements becomes paramount. Understanding the terminology associated with monitoring and sustainability ensures that teams can effectively communicate, implement, and maintain process improvements over time.
This comprehensive glossary explores the essential terms and concepts used during the Control Phase, providing practical examples and insights that will help you master the language of process control and sustainability. You might also enjoy reading about Reward and Recognition: Celebrating Team Success in the Lean Six Sigma Control Phase.
Understanding the Control Phase Foundation
Before diving into specific terminology, it is important to understand that the Control Phase serves as the bridge between project completion and long-term operational excellence. This phase ensures that the gains achieved during the Improve Phase are not temporary but become embedded in the organization’s standard operating procedures. You might also enjoy reading about How to Read and Interpret Control Charts Without Getting Confused: A Comprehensive Guide.
Control Plan
A Control Plan is a comprehensive document that outlines the specific methods, measurements, and actions required to maintain process improvements. This living document serves as a roadmap for sustaining gains and includes details about what to measure, how to measure it, when to measure it, and who is responsible for taking action when variations occur. You might also enjoy reading about Before and After Comparison: How to Document Improvement Results Effectively.
Example: In a customer service call center that reduced average call handling time from 8 minutes to 5 minutes, the Control Plan might specify that team leaders monitor call duration metrics daily, review weekly trend reports, and conduct monthly audits of call recordings to ensure quality has not been compromised while reducing time.
Statistical Process Control (SPC)
Statistical Process Control is a methodology that uses statistical methods to monitor and control processes. SPC relies on control charts to distinguish between common cause variation (inherent to the process) and special cause variation (resulting from specific, identifiable factors).
Sample Data Set: Consider a manufacturing line producing bottles. The target fill volume is 500ml with an acceptable range of 495ml to 505ml. Daily measurements over 20 days might show:
- Day 1-10: Readings between 498ml and 502ml (stable, within control limits)
- Day 11: Sudden reading of 490ml (special cause variation requiring investigation)
- Day 12-20: Return to readings between 497ml and 503ml after calibration adjustment
Control Chart
A Control Chart is a graphical tool used to study how a process changes over time. Data points are plotted in time order with a central line representing the average and upper and lower control limits that define the boundaries of expected variation.
Control charts help teams answer critical questions: Is the process stable? Are improvements holding? When should we intervene? There are various types of control charts including X-bar and R charts, Individual and Moving Range (I-MR) charts, P-charts, and C-charts, each suited to different types of data.
Example: A hospital tracking daily patient wait times might use an I-MR chart showing that after implementing a new triage system, the average wait time decreased from 45 minutes to 28 minutes, with control limits of 18 minutes (lower) and 38 minutes (upper). Any day showing wait times beyond these limits would trigger investigation.
Key Performance Indicators and Measurement Terms
Key Performance Indicator (KPI)
A Key Performance Indicator is a measurable value that demonstrates how effectively an organization is achieving key business objectives. In the Control Phase, KPIs serve as the vital signs of process health, providing early warning signals when performance begins to drift.
Example: An e-commerce company might track order fulfillment accuracy as a KPI. Before improvement, accuracy was 92%. After implementing a new verification system, it improved to 98.5%. The Control Phase monitors this KPI weekly to ensure it remains above the 97% threshold established during the Improve Phase.
Process Capability (Cp and Cpk)
Process Capability indices measure how well a process can produce output within specification limits. Cp measures potential capability assuming the process is perfectly centered, while Cpk measures actual capability considering how well the process is centered within specification limits.
Sample Calculation: A component manufacturing process has specifications of 50mm ± 5mm (USL=55mm, LSL=45mm). If the process mean is 50mm with a standard deviation of 1.5mm:
- Cp = (USL – LSL) / (6 × standard deviation) = (55 – 45) / (6 × 1.5) = 1.11
- This Cp value greater than 1.0 indicates the process is potentially capable
Baseline Metrics
Baseline Metrics represent the original performance measurements taken before improvements were implemented. These serve as the reference point for measuring success and validating that improvements have been sustained over time.
Example: A software development team measured their baseline defect rate at 8.3 defects per 1,000 lines of code before implementing code review practices. Six months after implementation, maintaining a rate of 2.1 defects per 1,000 lines demonstrates sustained improvement from the baseline.
Voice of the Customer (VOC)
Voice of the Customer represents the stated and unstated needs, requirements, and expectations of customers. In the Control Phase, ongoing VOC collection ensures that improvements continue to meet customer needs and helps identify new opportunities for enhancement.
Variation and Stability Terminology
Common Cause Variation
Common Cause Variation refers to the natural, inherent fluctuation present in all processes. This variation is predictable within statistical limits and results from the regular functioning of the process. It is also known as random variation or noise.
Example: In a bakery, slight variations in bread loaf weight (between 498g and 502g when targeting 500g) due to minor differences in ingredient measurement, oven temperature fluctuations, or humidity represent common cause variation.
Special Cause Variation
Special Cause Variation occurs due to specific, identifiable factors that are not inherent to the process. This variation is unpredictable, intermittent, and indicates that something unusual has affected the process. It is also called assignable cause variation.
Example: In the same bakery, if bread loaves suddenly weigh 450g due to a malfunctioning ingredient dispenser, this represents special cause variation requiring immediate investigation and correction.
Process Stability
Process Stability indicates that a process is performing predictably over time with only common cause variation present. A stable process has consistent performance and no trends, shifts, or unusual patterns in the data.
Upper Control Limit (UCL) and Lower Control Limit (LCL)
The Upper Control Limit and Lower Control Limit are statistical boundaries on a control chart, typically set at three standard deviations above and below the process mean. Points falling outside these limits signal special cause variation requiring investigation.
Sample Data: A customer service team monitoring response times might have:
- Mean response time: 4.2 hours
- Standard deviation: 0.8 hours
- UCL: 4.2 + (3 × 0.8) = 6.6 hours
- LCL: 4.2 – (3 × 0.8) = 1.8 hours
Documentation and Standard Operating Procedures
Standard Operating Procedure (SOP)
A Standard Operating Procedure is a detailed, written instruction document that outlines the steps required to complete a specific task or process consistently. SOPs are fundamental to sustaining improvements by ensuring everyone follows the improved process in the same way.
Example: After reducing patient intake errors by 75% through a new verification process, the hospital creates an SOP detailing the exact steps: verify patient ID using two identifiers, scan wristband, confirm allergy information in system, document verification in electronic health record. This ensures all staff follow the improved process consistently.
Work Instructions
Work Instructions are detailed, step-by-step guidelines that show exactly how to perform a specific task. These are typically more granular than SOPs and may include photos, diagrams, or videos to ensure clarity.
Process Documentation
Process Documentation encompasses all written materials that describe how a process works, including flowcharts, procedures, work instructions, and forms. Comprehensive documentation is essential for training new employees and maintaining consistency.
Training and Competency Terms
Training Matrix
A Training Matrix is a visual tool that shows which employees have been trained on which processes or procedures. This matrix helps ensure adequate coverage and identifies training gaps that could threaten sustainability.
Example: A manufacturing facility might maintain a matrix showing that 8 out of 12 operators are certified on the new quality inspection procedure, highlighting the need to train the remaining 4 operators to ensure sustainability when key employees are absent.
Competency Assessment
Competency Assessment is the process of verifying that employees can perform tasks correctly according to established standards. Regular assessments ensure that knowledge and skills are maintained over time.
Knowledge Transfer
Knowledge Transfer involves systematically sharing information, skills, and expertise from those who have it to those who need it. This is critical for sustainability when team members change roles or leave the organization.
Auditing and Compliance Terminology
Process Audit
A Process Audit is a systematic examination of a process to determine whether activities and results comply with planned arrangements and whether these arrangements are implemented effectively. Regular audits help detect process drift before it significantly impacts performance.
Example: A monthly audit of the improved order fulfillment process might review 50 randomly selected orders to verify that all steps in the new procedure were followed correctly. If compliance drops below 95%, corrective action is triggered.
Compliance Rate
Compliance Rate measures the percentage of time that established procedures are followed correctly. This metric directly correlates with sustainability, as non-compliance typically leads to performance degradation.
Corrective Action
Corrective Action refers to steps taken to eliminate the cause of a detected nonconformity or undesirable situation. In the Control Phase, having a clear corrective action process ensures that problems are addressed systematically before they undermine improvements.
Preventive Action
Preventive Action involves steps taken to eliminate the cause of a potential nonconformity or undesirable situation before it occurs. This proactive approach helps maintain process improvements by addressing risks before they materialize.
Response and Escalation Terms
Response Plan
A Response Plan outlines the specific actions to be taken when process performance falls outside acceptable limits. This plan includes trigger points, responsible parties, and step-by-step actions to return the process to control.
Example: An online retailer’s Response Plan for website downtime might specify:
- If downtime exceeds 5 minutes: Notify IT supervisor
- If downtime exceeds 15 minutes: Activate backup server and notify customers via social media
- If downtime exceeds 30 minutes: Escalate to CTO and activate crisis communication plan
Escalation Protocol
An Escalation Protocol defines when and how issues should be elevated to higher levels of management. Clear protocols ensure that significant problems receive appropriate attention quickly.
Trigger Point
A Trigger Point is a predefined threshold that, when crossed, automatically initiates a specific action or response. Trigger points enable rapid response to process variation before small issues become major problems.
Sustainability and Continuous Improvement
Process Ownership
Process Ownership assigns clear responsibility for the ongoing performance and improvement of a process to a specific individual. The process owner ensures that controls remain effective and that the process continues to meet business objectives.
Sustainability Plan
A Sustainability Plan is a comprehensive strategy for maintaining improvements over the long term. This plan addresses potential challenges to sustainability and outlines specific actions to ensure gains are not lost.
Example: After reducing invoice processing time by 40%, the accounting department’s Sustainability Plan includes quarterly training refreshers, monthly performance reviews, semi-annual process audits, and annual reviews of the control plan to incorporate lessons learned.
Continuous Monitoring
Continuous Monitoring involves the ongoing collection and analysis of process data to detect changes in performance. This vigilant approach enables early detection of process drift and rapid corrective action.
Performance Dashboard
A Performance Dashboard is a visual display of key metrics that provides at-a-glance information about process performance. Effective dashboards make it easy to spot trends and anomalies quickly.
Example: A logistics company’s dashboard might display:
- On-time delivery rate: 96.3% (target: 95%, green status)
- Average delivery time: 2.1 days (target: 2.0 days, yellow status)
- Damaged shipments: 0.8% (target: 1.0%, green status)
- Customer satisfaction score: 4.6/5.0 (target: 4.5/5.0, green status)
Process Drift
Process Drift occurs when a process gradually deviates from its intended state over time. This subtle degradation can go unnoticed without proper monitoring and often results from inadequate controls, insufficient training, or lack of process ownership.
Advanced Control Concepts
Control Strategy
A Control Strategy is the overall approach used to maintain process performance within acceptable limits. This strategy may include a combination of monitoring tools, response protocols, training programs, and audit schedules tailored to the specific process and organizational context.
Mistake Proofing (Poka-Yoke)
Mistake Proofing, or Poka-Yoke, involves designing processes or systems to prevent errors from occurring or make errors immediately obvious when they do occur. This proactive control method reduces reliance on detection and inspection.
Example: A pharmaceutical packaging line uses sensors that detect whether the correct number of tablets are in each bottle. If the count is incorrect, the bottle is automatically rejected and an alert is sent to the operator, preventing defective products from reaching customers.
Visual Management
Visual Management uses visual signals, displays, and controls to communicate important information quickly and clearly. In the Control Phase, visual management makes process status transparent and enables rapid identification of abnormalities.
Control Limits versus Specification Limits
Understanding the distinction between Control Limits and Specification Limits is crucial. Control limits are calculated from process data and represent the voice of the process (what the process is actually doing), while specification limits are determined by customer requirements and represent the voice of the customer (what the process should do).
Example: A paint manufacturer might have:
- Specification limits for color matching: ΔE ≤ 2.0 (customer requirement)
- Control limits based on actual process performance: UCL = 1.6, LCL = 0.4
- The process is both in control (within control limits) and capable (within specification limits)
Implementing Effective Control Phase Practices
Understanding this terminology is just the beginning. Effective implementation requires commitment, discipline, and organizational support. Successful organizations integrate Control Phase activities into daily operations rather than treating them as separate project activities.
Consider a real-world scenario: A financial services company reduced loan processing time from 14 days to 7 days through process
