Measurement System Analysis (MSA)

When to Activate This Skill

• "Conduct gage R&R study"
• "Evaluate measurement system for [gage/characteristic]"
• "Calculate %GR&R"
• "Perform attribute agreement analysis"
• "What's the ndc for this gage?"
• "Is this measurement system acceptable?"
• "MSA requirements for [characteristic]"

Purpose of MSA

MSA determines how much of the observed process variation is due to the measurement system rather than the actual process. Before making decisions based on measurement data, we must verify the measurement system is adequate.

Why MSA Matters

Without MSA:

• "Good" parts may be rejected
• "Bad" parts may be accepted
• Process capability may be understated
• SPC decisions may be wrong
• Customer complaints may result

With MSA:

• Measurement confidence established
• Gage selection validated
• Training effectiveness verified
• Calibration adequacy confirmed

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Types of MSA Studies

Variable MSA (Gage R&R)

For measurements that produce numerical data (dimensions, weight, temperature, etc.)

Study Type	Purpose	Method
Repeatability	Same operator, same gage, same part, multiple measurements	Single operator, 10+ measurements
Reproducibility	Different operators, same gage, same parts	Multiple operators measure same parts
Gage R&R	Combined repeatability and reproducibility	Standard study
Bias	Difference between measured and true value	Compare to master
Linearity	Bias across measurement range	Multiple references
Stability	Variation over time	Control chart on master

Attribute MSA

For measurements that produce pass/fail, good/bad, or categorical results

Study Type	Purpose	Method
Attribute Agreement	Operator consistency and accuracy	Multiple operators, multiple trials
Kappa	Agreement beyond chance	Statistical calculation
Effectiveness	Correct decisions vs. actual status	Reference evaluation

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Variable Gage R&R Study

Study Design (Standard AIAG)

Parameter	Minimum	Preferred	Notes
Operators	2	3	Include typical operators
Parts	5	10	Represent process variation
Trials	2	3	Repeat measurements
Total readings	20	30-90	More = better discrimination

Study Execution

1. Select parts - Cover full range of process variation
2. Number parts - Hidden from operator view
3. Randomize - Operator doesn't know which part
4. Measure - Each operator measures all parts, multiple trials
5. Record - Document all measurements
6. Analyze - Calculate %GR&R, ndc

Acceptance Criteria

Metric	Acceptable	Marginal	Unacceptable
%GR&R (vs Process)	<10%	10-30%	>30%
%GR&R (vs Tolerance)	<10%	10-30%	>30%
ndc (Number of Distinct Categories)	≥5	3-4	<3

Interpretation

%GR&R <10%:

• Measurement system acceptable
• Can distinguish part-to-part variation
• Suitable for SPC

%GR&R 10-30%:

• May be acceptable for non-critical applications
• Requires customer approval for critical characteristics
• Consider improvement actions

%GR&R >30%:

• Measurement system not acceptable
• Must improve before use
• Consider: different gage, training, environment

ndc (Number of Distinct Categories):

• Represents how many groups the gage can distinguish
• ndc ≥5 required for variable data
• ndc <5 means gage acts more like attribute (good/bad only)

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Gage R&R Calculations

ANOVA Method (Preferred)

Analysis of Variance separates total variation into:

• Part-to-part variation
• Operator variation
• Operator × Part interaction
• Repeatability (equipment)
• Reproducibility (operator)

Range Method (X-bar/R)

Simpler calculation, widely used:

Repeatability (EV) = R̄ × K₁
Where: R̄ = average range across all operators
       K₁ = factor based on number of trials

Reproducibility (AV) = √[(X̄diff × K₂)² - (EV²/nr)]
Where: X̄diff = range of operator averages
       K₂ = factor based on number of operators
       n = number of parts
       r = number of trials

GR&R = √(EV² + AV²)

%GR&R = (GR&R / TV) × 100
Where: TV = Total Variation = √(GR&R² + PV²)
       PV = Part Variation

ndc = 1.41 × (PV / GR&R)

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Attribute Agreement Analysis

When to Use

• Go/No-go gages
• Visual inspection
• Pass/fail tests
• Any categorical decision

Study Design

Parameter	Minimum	Preferred
Appraisers	2	3
Samples	20	30-50
Trials	2	3
Sample mix	Include borderline	50% good, 50% bad, include borderline

Key Metrics

Metric	Description	Target
Within Appraiser Agreement	Self-consistency	≥90%
Between Appraiser Agreement	Appraiser vs. Appraiser	≥90%
Appraiser vs. Standard	Appraiser vs. Reference	≥90%
Kappa	Agreement beyond chance	≥0.75

Kappa Interpretation

Kappa Value	Interpretation
<0.20	Poor agreement
0.21-0.40	Fair agreement
0.41-0.60	Moderate agreement
0.61-0.80	Substantial agreement
0.81-1.00	Almost perfect agreement

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Other MSA Studies

Bias Study

Measures systematic error (difference from true value)

Method:

1. Obtain reference standard (known true value)
2. Measure standard multiple times (≥10)
3. Calculate average of measurements
4. Bias = Average - Reference value

Acceptance: Bias ≈ 0 or within calibration tolerance

Linearity Study

Measures bias across the measurement range

Method:

1. Select 5+ reference standards across range
2. Measure each standard multiple times
3. Plot bias vs. reference value
4. Fit regression line

Acceptance: Linearity (slope × Process Variation) <5%

Stability Study

Measures variation over time

Method:

1. Select stable reference part/standard
2. Measure periodically (daily, weekly)
3. Plot on control chart
4. Monitor for trends or out-of-control

Acceptance: Stable control chart, no trends

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MSA Requirements by Application

IATF 16949 Requirements (7.1.5.1.1)

• MSA required for all measurement systems in Control Plan
• Shall include study guidance and acceptance criteria
• Alternative methods may be used with customer approval

Application Guidelines

Characteristic	Required MSA	Criteria
Critical (CC)	Gage R&R (variable) or Attribute Agreement	%GR&R <10%
Significant (SC)	Gage R&R (variable) or Attribute Agreement	%GR&R <30%
Standard	Gage R&R recommended	%GR&R <30%
SPC-monitored	Gage R&R required	ndc ≥5

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Common MSA Issues and Solutions

Issue	Likely Cause	Solution
High repeatability	Gage resolution, condition	Better gage, calibrate, repair
High reproducibility	Training, technique	Standardize method, train
High interaction	Operator-dependent method	Simplify method, fixture
Poor ndc	Gage can't see variation	More sensitive gage
Low Kappa	Ambiguous criteria	Define clearer standards
Bias	Calibration, wear	Recalibrate, adjust

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Output Format

When generating MSA content:

# MSA Study Report

## Study Information
| Field | Value |
|-------|-------|
| **Study Type** | Gage R&R / Attribute Agreement |
| **Gage ID** | [ID] |
| **Gage Description** | [Type, range, resolution] |
| **Characteristic** | [What is measured] |
| **Specification** | [Tolerance] |
| **Study Date** | [Date] |
| **Conducted By** | [Name] |

## Study Parameters
| Parameter | Value |
|-----------|-------|
| Operators | [Number and names] |
| Parts | [Number] |
| Trials | [Number] |
| Total measurements | [Count] |

## Results
| Metric | Value | Acceptance | Status |
|--------|-------|------------|--------|
| %GR&R | [X]% | <10% / <30% | PASS/FAIL |
| ndc | [X] | ≥5 | PASS/FAIL |
| Repeatability | [X]% | - | - |
| Reproducibility | [X]% | - | - |

## Conclusion
[ACCEPTABLE / MARGINAL / UNACCEPTABLE]

## Actions (if required)
- [Action items]

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Integration with Related Skills

ControlPlan

All gages in Control Plan require MSA:

• Variable gages: Gage R&R
• Attribute gages: Attribute agreement
• MSA status verified before production

Load: read ~/.claude/skills/Controlplan/SKILL.md

SPC

SPC validity depends on MSA:

• ndc ≥5 required for variable SPC
• Measurement variation affects control limits
• Poor MSA = poor SPC decisions

Load: read ~/.claude/skills/Spc/SKILL.md

AutomotiveManufacturing

MSA supports work instruction development:

• Measurement methods documented
• Gage identification required
• Operator training verified

Load: read ~/.claude/skills/Automotivemanufacturing/SKILL.md

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Supplementary Resources

For detailed guidance: read ~/.claude/skills/Msa/CLAUDE.md

For study templates: ls ~/.claude/skills/Msa/templates/

For acceptance criteria: read ~/.claude/skills/Msa/reference/acceptance-criteria.md

For calculation formulas: read ~/.claude/skills/Msa/reference/calculation-formulas.md