Measurement System Reanalysis: A Comprehensive Guide for Six Sigma Black Belt

Measurement System Reanalysis in Six Sigma Black Belt Control Phase

Why Measurement System Reanalysis is Important

Measurement System Reanalysis is critical in the Control phase of Six Sigma because it ensures that the improvements made during the Improve phase remain valid and sustainable. Here's why it matters:

• Validates Continued Accuracy: Over time, measurement systems can drift or degrade. Reanalysis confirms that your measurement system remains accurate and capable of detecting process changes.
• Ensures Process Control Validity: Control charts and monitoring systems rely on accurate measurements. Without reanalysis, you may be making decisions based on faulty data.
• Prevents False Improvements: Sometimes apparent process improvements are actually artifacts of measurement system changes rather than real process improvements.
• Maintains Statistical Integrity: Six Sigma improvements depend on reliable data. Reanalysis protects the credibility of your project results.
• Compliance and Sustainability: Regular measurement system verification is often required for quality standards (ISO, automotive, pharmaceutical) and helps sustain gains long-term.

What is Measurement System Reanalysis?

Measurement System Reanalysis is the periodic reassessment of a measurement system's performance to ensure it continues to meet the requirements established during the Measure phase. It involves repeating key measurement system analysis (MSA) studies to verify that:

• Accuracy (bias) has not drifted
• Precision (repeatability and reproducibility) remains acceptable
• Linearity across the measurement range is maintained
• Stability over time is consistent
• The measurement system continues to discriminate between acceptable and unacceptable parts

During the Control phase, reanalysis serves as a verification checkpoint rather than a comprehensive new study. It typically involves:

• Re-running Gage R&R studies at predetermined intervals
• Monitoring measurement system drift trends
• Confirming operator consistency and training effectiveness
• Validating that calibration procedures are effective
• Checking for environmental factor changes affecting measurements

How Measurement System Reanalysis Works

Step 1: Establish Reanalysis Schedule

• Determine frequency based on process criticality, historical stability, and regulatory requirements
• Typical frequencies: quarterly, semi-annually, or annually
• High-risk processes may require monthly verification
• Document in control plan with clear triggers and responsibilities

Step 2: Select Reference Parts/Standards

• Use the same reference parts from the original MSA if possible
• Alternatively, use certified standards with known values
• Ensure parts represent the full range of the measurement system
• Include parts near process specifications and control limits

Step 3: Conduct Abbreviated Gage R&R

• For reanalysis, you may use fewer replicates than the original study (e.g., 2 instead of 3)
• Sample size typically: 5-10 parts × 2-3 operators × 2 replicates
• Randomize measurement order to prevent pattern bias
• Include both experienced and new operators when possible

Step 4: Analyze Results

• Calculate %GR&R: Should remain below 30% (ideally under 10-20% for critical characteristics)
• Compare to baseline: Identify any drift from original MSA results
• Evaluate by source: Determine if drift is due to repeatability, reproducibility, or both
• Statistical significance: Use control charts to trend measurement system performance

Step 5: Check for Specific Issues

• Bias/Accuracy: Verify measured values still match true standards
• Linearity: Confirm accuracy is consistent across measurement range
• Stability: Review trend charts for equipment drift over time
• Discrimination: Confirm ability to differentiate parts meeting vs. failing specifications

Step 6: Take Corrective Action if Needed

• If %GR&R has increased significantly, investigate root causes
• Potential actions: recalibration, operator retraining, equipment maintenance, environmental controls
• Re-run study after corrections to verify improvement
• Document all findings and actions taken

Step 7: Update Control Plan and Documentation

• Record reanalysis results with date and analyst name
• Update control plan with current measurement system status
• Communicate results to relevant teams
• Schedule next reanalysis date

Key Metrics in Measurement System Reanalysis

%GR&R (Gage Repeatability & Reproducibility): Percentage of process tolerance or natural process variation consumed by measurement system

• Below 10%: Excellent measurement system
• 10-20%: Acceptable (marginal for critical characteristics)
• 20-30%: Marginal (should improve if possible)
• Above 30%: Unacceptable, system needs improvement

Bias (Accuracy): Systematic error in measurements. Reanalysis should show stable bias or trending within acceptable limits.

Discrimination Ratio: Number of distinct categories the measurement system can reliably detect. Ideally ≥ 5 categories.

Repeatability: Variation in measurements by same operator with same equipment. Should remain consistent with original study.

Reproducibility: Variation between different operators or conditions. Indicates training consistency and standardization effectiveness.

Common Issues Found During Reanalysis

• Measurement Drift: Equipment has shifted from calibration, usually addressed through recalibration
• Operator Degradation: Operators have lost proficiency, requiring retraining
• Environmental Changes: Temperature, humidity, vibration changes affecting sensitive measurements
• Equipment Wear: Normal wear of measurement devices reducing accuracy
• Increased Variation: Process changes have increased spread, making discrimination harder
• Specification Creep: Process now operating closer to limits, requiring better measurement capability

Exam Tips: Answering Questions on Measurement System Reanalysis

Tip 1: Understand the Purpose in Context

• Reanalysis occurs in the Control phase, not earlier phases
• It verifies that improvements remain valid and systems remain stable
• Distinguish it from the initial MSA Measure phase study (reanalysis is periodic verification)
• When an exam question mentions "ongoing verification" or "periodic checking" of measurement systems in Control phase, think Reanalysis

Tip 2: Know the Right Frequency and Sample Size

• Exam questions often ask about reanalysis schedules
• Frequency depends on: process criticality, historical stability, regulatory requirements, and risk
• For reanalysis (not original study), abbreviated versions are acceptable: fewer operators, fewer replicates, fewer parts
• Original study might be 10 parts × 3 operators × 3 replicates; reanalysis might be 5 parts × 2-3 operators × 2 replicates

Tip 3: Recognize Reanalysis vs. Other MSA Activities

• Initial Gage R&R (Measure phase): Comprehensive assessment before improving
• Reanalysis (Control phase): Periodic verification to confirm stability
• Capability Analysis: Determining process capability, not measurement system quality
• Validation Study: Ensuring new measurement method works before use
• Exam questions may mix these—read carefully for keywords like "Control phase," "ongoing," "periodic," or "periodic verification"

Tip 4: Know Acceptable Criteria

• Standard threshold: %GR&R should be <30% (better if <20% or <10%)
• If reanalysis shows %GR&R has degraded from original 15% to 28%, this is concerning
• Questions may ask: "What does increasing %GR&R indicate?" → Equipment drift, operator drift, or environmental changes
• If reanalysis shows improvement from original, the question likely wants you to identify the corrective action taken

Tip 5: Understand Corrective Actions

• If reproducibility degraded: operator retraining needed
• If repeatability degraded: equipment calibration or maintenance needed
• If bias increased: check for calibration drift or environmental changes
• If discrimination ratio decreased: may need more precise equipment
• Exam questions often present a reanalysis result and ask "What should be done next?" — answer with specific corrective action based on what metric degraded

Tip 6: Connect to Control Plan and SOP

• Reanalysis findings should update the control plan
• Measurement system procedures (SOP) should include reanalysis frequency and methods
• Questions may ask: "Where should reanalysis schedule be documented?" → Answer: Control Plan
• Questions may ask: "Who is responsible for reanalysis?" → Answer: Process owner or quality/measurement system team member specified in control plan

Tip 7: Calculate and Interpret %GR&R Correctly

• Formula: %GR&R = (GR&R Standard Deviation / Process Tolerance or Natural Process Variation) × 100
• Exam may give you Gage R&R value (e.g., 2.5) and ask to calculate %GR&R
• You'll need process tolerance (USL - LSL) or process variation (6σ)
• Example: If GR&R = 3.2 and tolerance = 20, then %GR&R = (3.2/20) × 100 = 16%
• Then interpret: 16% is acceptable but marginal for critical characteristics

Tip 8: Recognize Red Flags

• %GR&R increasing over time → Equipment degradation or operator drift
• %GR&R varies significantly between operators → Need reproducibility improvement (training)
• %GR&R varies between shifts or times → Environmental factors at play
• Discrimination ratio decreasing → Measurement system becoming less capable
• Questions may present trend data and ask "What does this pattern suggest?" — identify the root cause

Tip 9: Answer Format for Exam Questions

• Multiple Choice: Look for answer mentioning "periodic verification," "Control phase," "sustainability," or "ongoing monitoring"
• Short Answer: Structure as: 1) What is being measured, 2) Why it's important in Control phase, 3) What standards/criteria apply, 4) What action if findings are bad
• Scenario-based: Given a reanalysis result, identify: the problem (what metric changed), the likely cause (why), and the solution (what to do)

Tip 10: Common Exam Question Patterns

• "After 6 months in production, a Black Belt performs a Gage R&R reanalysis. The %GR&R has increased from 8% to 22%. What is the most likely reason?" → Identify the most probable cause (often equipment drift or operator training loss)
• "The control plan should specify the measurement system reanalysis frequency. What factors should determine this?" → Answer: process criticality, regulatory requirements, historical stability, risk level
• "During reanalysis, which measurement system parameter is most sensitive to operator training?" → Answer: Reproducibility
• "A reanalysis shows repeatability is excellent but reproducibility is poor. What corrective action is most appropriate?" → Answer: Operator training and standardization of procedures
• "What is the primary goal of measurement system reanalysis in the Control phase?" → Answer: Verify that the measurement system remains capable and stable to ensure process control validity and sustain improvements

Summary

Measurement System Reanalysis is a critical Control phase activity that periodically verifies measurement system performance to ensure continued accuracy, precision, and capability. By establishing a reanalysis schedule, conducting abbreviated studies at regular intervals, analyzing results against acceptance criteria, and taking corrective action when needed, Black Belts ensure that process improvements are based on reliable data and that control systems function effectively. Understanding reanalysis, its frequency, acceptable criteria, and appropriate corrective actions is essential for Black Belt exam success and real-world project sustainability.