Msa Gauge Rr

Measurement System Analysis (MSA) / Gauge R&R

When to use

Use this skill when:

• Qualifying a measurement system for a new part or process (PPAP requirement)
• Interpreting Gauge R&R results — is this gauge acceptable?
• Auditing a supplier's MSA study for correctness and adequacy
• Selecting the right study type for a given measurement situation
• Investigating a quality problem where measurement error may be a factor
• Responding to a customer request for MSA data on a specific characteristic

Prerequisites

• The characteristic to be measured (product or process)
• The gauge or measurement system to be studied
• Production parts spanning the expected process variation (10 parts minimum)
• At least 2 trained appraisers who normally perform the measurement
• The specification (tolerance) for the characteristic

Workflow

Step 1 — Select the MSA study type

| Study type | When to use | |-----------|-------------| | Gauge R&R (crossed) | Variable data, 2–3 appraisers, each measures all parts (most common) | | Gauge R&R (nested) | Variable data, parts are destroyed during measurement (e.g., tensile test) | | Attribute MSA | Pass/fail, go/no-go, visual inspection — data is not a number | | Bias study | Accuracy of a single gauge vs. a reference standard | | Linearity study | Whether gauge accuracy is consistent across its measurement range | | Stability study | Whether gauge accuracy drifts over time |

For PPAP: Crossed Gauge R&R is required for all variable gauges on special characteristics.

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Step 2 — Crossed Gauge R&R study procedure

Setup:

• Minimum 10 parts selected to represent the full process variation range (not cherry-picked from centre of tolerance)
• Minimum 2 appraisers (3 preferred)
• Minimum 2 trials per appraiser per part (3 preferred)
• Parts randomly numbered and labelled — appraisers must not see each other's measurements
• Measurement conditions must match normal production conditions

Standard study design:

• 10 parts × 3 appraisers × 2 trials = 60 measurements
• 10 parts × 2 appraisers × 3 trials = 60 measurements

Execution:

1. Appraiser A measures all 10 parts in random order — record results
2. Appraiser B measures all 10 parts in random order — record results
3. Appraiser C measures all 10 parts in random order — record results
4. Repeat the cycle for trial 2 (and trial 3 if applicable)
5. Appraisers must not see their own previous results or other appraisers' results during the study

Do NOT:

• Allow appraisers to adjust the gauge between trials
• Use parts that are all near the nominal value (no spread)
• Record only one trial (single measurement per appraiser per part is insufficient)

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Step 3 — Interpret the results

%GRR (Gauge R&R as % of Total Variation or % of Tolerance)

| %GRR | Interpretation | Decision | |------|---------------|---------| | 30% | Unacceptable | ❌ Gauge not suitable — investigate and improve before use in production |

Two calculation methods:

• % of Study Variation (% of TV): GRR / Total Variation × 100 — preferred when process is in control
• % of Tolerance: GRR / Tolerance × 100 — use when process capability is unknown or for attribute gauging

For PPAP, %GRR AV: investigate gauge (calibration, maintenance, resolution) If AV > EV: investigate training, measurement procedure, gauge fixture/setup

When %GRR > 30% — improving the measurement system

A result >30% means the gauge is not suitable for production use. Do not proceed to PPAP — investigate and retest. Common root causes and actions:

| Root cause (high EV) | Action | |---------------------|--------| | Gauge resolution too coarse | Replace with a gauge of finer resolution (rule: resolution ≤ 10% of tolerance) | | Gauge worn or damaged | Inspect, recalibrate, or replace the gauge | | Environmental interference (vibration, temperature) | Move measurement to a stable environment; add fixture if needed | | Inconsistent part fixturing | Design a repeatable fixture or measurement aid |

| Root cause (high AV) | Action | |---------------------|--------| | Measurement technique varies by appraiser | Develop a standardised measurement instruction (WI with photos/video) | | Gauge difficult to read or position | Redesign fixture; add a datum locator; use a self-positioning gauge | | Training gap | Retrain all appraisers using the standardised measurement WI; repeat the study |

After implementing improvements: re-run the full study. Do not accept a %GRR > 30% result with a customer waiver unless the characteristic is non-critical and the customer explicitly agrees in writing.

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Step 4 — Attribute MSA (pass/fail gauges)

For go/no-go gauges, visual inspection, and any pass/fail decision:

Expanded attribute study (recommended for PPAP):

• 50 parts spanning the full range (include parts near the accept/reject boundary)
• 3 appraisers × 3 trials per part = 450 measurements
• Calculate % agreement (within appraiser and between appraisers)
• Calculate kappa statistic (Cohen's kappa ≥ 0.9 is acceptable)

Short method (minimum acceptable):

• 20 parts × 2 appraisers × 2 trials
• 90% agreement within and between appraisers required
• Include borderline parts near the specification limit

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Step 5 — Audit an existing MSA study

When reviewing a supplier's or internal MSA, check:

• [ ] Parts selected represent the full process variation range (not all near nominal)
• [ ] Minimum 10 parts, 2 appraisers, 2 trials confirmed
• [ ] Appraisers did not see each other's or their own previous results
• [ ] %GRR result stated clearly (tolerance method or study variation method — which one?)
• [ ] ndc ≥ 5 for all special characteristics
• [ ] EV vs. AV breakdown analysed and interpreted
• [ ] %GRR 30% submitted in PPAP without explanation or customer waiver
• Gauge calibration expired at time of MSA study — study is invalid
• ndc "How would you like to receive the output?

> A — Structured Markdown (formatted tables and sections, ready to copy) > B — Plain tables (simplified structure for Excel or Word) > C — Narrative report (flowing text for a formal document or email) > > Default: A."

Adapt all output sections to the chosen format. If the platform or session context already defines a format preference, skip this question.

Changelog

| Version | Date | Author | Change | |---------|------|--------|--------| | 1.0 | 2026-06-06 | @RBraga01 | Initial release | | 1.1 | 2026-06-06 | @migmcc | Added improvement guidance for %GRR > 30% — root cause table for high EV and high AV with corrective actions |

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: RBraga01
• Source: RBraga01/Quality-Engineering-Skills
• License: MIT
• Homepage: https://rbraga01.github.io/Quality-Engineering-Skills

Install and usage instructions live in the source repository linked above.