Gage Repeatability and Reproducibility | Gage R&R | GRR Study

What is Gage Repeatability and Reproducibility Study?

→ Gage Repeatability and Reproducibility is known as Gage R&R Study.

→ Sometimes, it is also called a GRR Study.

→ It helps to determine how much variation comes from the measuring device and from different operators.

→ A Gage R&R study is a foundational tool in Measurement Systems Analysis (MSA).

→ This tool evaluates whether a measurement system is precise enough to determine if parts are within acceptable quality limits or out of the limit.

Table of Contents:

• What is Gage R&R Study?
• Why to Conduct a Gage R&R Study?
• When to Conduct a Gage R&R Study?
• Fundamentals of Gage Repeatability and Reproducibility Study
• GRR Study Formulas
• Repeatability - Equipment Variation (EV)
• Reproducibility - Appraiser Variation (AV)
• Gage Repeatability & Reproducibility (GRR)
• Part Variation (PV)
• Total Variation (TV)
• Contribution of Appraiser Variation (AV) (%)
• Contribution of Equipment Variation (EV) (%)
• Contribution of GRR (%)
• Number of Distinct Categories (NDC)
• Example of Gage R&R Study
• 5 Steps to Conduct GRR Study
• Gage R&R Acceptance Criteria
• NDC Acceptance Criteria
• How to Improve Measurement System?
• How to Improve Gage Repeatability?
• How to Improve Gage Reproducibility?
• How to Reduce Part Variation?
• Final Takeaways
• Benefits
• Conclusion

Why to Conduct a Gage R&R Study?

→ A GRR study is crucial for evaluating the reliability and precision of a data collection.

⏩Key reasons why GRR study is conducted:

• Ensure measurement system reliability and suitability
• Identify sources of variation
• Improve process capability
• Decision-making
• Compliance with industry standards
• Reduce waste and costs
• Optimize operator training and equipment selection

 

When to Conduct a Gage R&R Study?

→ A GRR study should be conducted whenever there is a need to assess and validate the reliability of data collection.

⏩Key situations when a GRR study is necessary are:

• After equipment calibration or maintenance
• When a new operator is introduced
• As part of a six sigma or quality improvement initiative
• During process qualification or validation
• When a process change affects measurement
• Before conducting capability studies such as Cp, Cpk, Pp, Ppk study

Fundamentals of Gage R&R Study:

→ A Gage R&R Study is a statistical tool used in MSA.

→ It is used to determine the amount of variation in the measurement system.

→ This method is commonly used in almost all manufacturing industries.

⏩GRR study is used for:

• Comparison of different measuring devices
• Improving the performance of measuring instruments
• Comparison for measuring the instrument before and after repair
• Quantifying the process variation
• Approve the acceptance level for a production process

Gage R&R Study Formulas:

→ A GRR study identifies variation by calculating repeatability and reproducibility.

→ Now we will discuss the different key elements of this study.

⏩Key Elements of Gage R&R Study:

1. Repeatability - Equipment Variation (EV)
2. Reproducibility - Appraiser Variation (AV)
3. Gage Repeatability & Reproducibility (GRR)
4. Part Variation (PV)
5. Total Variation (TV)
6. Contribution of Appraiser Variation (AV) (%)
7. Contribution of Equipment Variation (EV) (%)
8. Contribution of GRR (%)
9. Number of Distinct Categories (NDC)

→ Now we will learn all formulas with examples.

01. Repeatability - Equipment Variation (EV)

→ Repeatability refers to the variation in measurements when the same operator measures the same part multiple times using the same gage under the same conditions.

→ It represents the consistency of a single operator measuring the same part multiple times using the same gage.

→ In other words, we can say that weather gage produce the same results under identical conditions or not?

→ In simple words, we can say that it is the variation of equipment or instrument (EV).

→ Repeatability measures when measuring the same part (and the same characteristic) using the same instrument more than one time.

→ If repeatability is high then we can say that the equipment is likely faulty, damaged, or insufficiently sensitive for the application.

➨ Where K1 is a constant that depends on the number of trials.

     ⇢ For 2 trials, K1 is 0.8862.

     ⇢ For 3 trials, K1 is 0.5908.

02. Reproducibility - Appraiser Variation (AV):

→ A "Bad" Result Means: High reproducibility often points to a need for better operator training, clearer measurement procedures (SOPs), or better-designed measurement fixtures.

→ Reproducibility measures the variation in measurements when different operators use the same gage to measure the same part under the same conditions.

→ It represents the variation in average measurements between operators.

→ In simple words, we can say that it is an operator/appraiser variation (AV).

→ Reproducibility measures when the different appraisers measure the same characteristic on the same part with the same instrument more than one time.

→ If the reproducibility is high then we need to provide better operator training, provide clear SOPs, and good measurement fixtures.

➨ Where K2 is a constant that depends on the number of appraisers (oerators).

     ⇢ For 2 appraisers, K2 is 0.7071

     ⇢ For 3 appraisers, K2 is 0.5231

03. Gage Repeatability & Reproducibility:

→ GRR Study is a statistical method used to assess the variation in a measurement system.

→ It helps determine whether the system is accurate, consistent, and reliable for quality control in manufacturing and other industries.

→ It is the combination of AV and EV.

→ GRR is the measurement of the entire system's precision.

→ Also, we can say that the vector sum of EV and AV, the calculation is given below.

04. Part Variation (PV):

→ PV represents the natural variation in the different parts chosen for the study.

→ It represent the dimensional difference that we want to study.

→ It is a critical component of a Gage Repeatability & Reproducibility study.

→ Because it indicates whether the measurement system can differentiate between different parts effectively or not.

→ We need to ensure that the part variation should be higher that the gage variation.

→ That means the gage should be sensitive enough to detect the real difference between parts.

→ The PV is calculated by multiplying the range of the part averages (Rp) by a constant K3.

→ K3 depends on the number of parts.

➨ Some other values of K3 are menitnoed below table:

05. Total Variation (TV):

→ TV represents the overall variation.

→ It is the combination of variation due to the Gage R&R and the actual variation in the parts.

→ We can calculate the TV by doing a vector sum of  PV and GRR.

06. Contribution of Appraiser Variation (AV) (%):

→ AV represent the appraiser variation and the contribution is the % of AV.

→ The formula is mentioned below.

07. Contribution of Equipment Variation (EV) (%):

→ EV represent the equipment variation and the contribution is the % of EV.

→ The formula is mentioned below.

08. Contribution of GRR (%):

→ The contribution of GRR helps determine how much of the total variation is due to the measurement process itself.

⏩Interpretation Criteria (based on MSA standards):

GRR < 10%: Acceptable.

GRR is between 10% to 30%: The system may be acceptable based on the importance of the application, cost of improvement, etc.

GRR > 30%: Unacceptable.

09. Number of Distinct Categories (NDC):

→ Number of Distinct Categories (NDC) is a measure of how well a measurement system can differentiate between different parts.

→ It helps determine whether the measurement system is capable of distinguishing meaningful differences in the process.

→ A measurement system must have an NDC of 5 or more to be considered acceptable for distinguishing parts within a process.

→ An NDC of 1 means the gage cannot distinguish one part from another.

→ A high NDC means the gage has high resolution and can easily detect differences between parts.

Example of Gage R&R Study:

→ A Gage R&R Study is performed to evaluate the amount of variation in a system caused by the measurement device and the operators using it.

→ This ensures the measurement system is accurate, precise, and reliable for quality control.

→ Now we will learn a step-by-step example for better understanding.

→ In this example we will take plastic pin for GRR Study.

→ Refer to the below mentioned basic details of the study:

⏩5 Steps to Conduct a Gage R&R Study:

1. Define the Purpose of the Gage R&R Study
2. Select Parts, Operators, Measurement Method, Instrument
3. Conduct the Measurements and Record Data
4. Analyze the Results
5. Interpretation of the Results

Step 1: Define the Purpose of the Gage R&R Study:

→ Before starting this study, we need to identify the following points.

→ Why the study is being conducted, such as new gage, operator training, process improvement, etc.

→ The measurement system being evaluated.

→ The tolerance/specifications for measurement.

Step 2: Select Parts, Operators, Measurement Method, Instrument:

→ Now in the next step, we need to determine the following things.

→ Parts: Choose at least 10 parts that represent the full range of process variation.

→ Appraisers: Select at least 3 operators who will perform the measurements.

→ Repetitions: Each operator should measure each part at least 2–3 times.

→ Measurement Method: Define a standardized measurement procedure to ensure consistency.

→ Instrument: Calibrate the instrument or Verify that the last calibration date is valid.

👉 Additional Notes:

→ The number of parts (n) must be greater than or equal to 5.

→ The appraisers (k) must be greater than 2.

→ The number of trials (r) must be greater than or equal to two.

→ In addition, the n*k should be greater than 15.

→ This gives more confidence in the results.

 

Step 3: Conduct the Measurements and Record Data:

→ Now we need to label the parts from 1 to n and we have to designate the appraisers and A, B, C, etc.

→ During study, we have to take care of one thing that the appraisers must not be aware of these things, otherwise we can not get a proper result.

→ The parts must be run in random order.

→ All appraisers measure the reading each part 3 times.

→ Example: Trial1 – first in order, Trial2 - second in reverse order, Trial3 - third random.

→ Start with appraiser A - measures the parts in random order.

→ This process continues for each appraiser.

→ This cycle is continued until you have completed all trials.

→ Be sure that an appraiser cannot see his/her results from previous trials.

→ Record data in sheet.

Step 4: Analyze the Results:

→ Analyze data in the Gage R&R worksheet.

→ In this analysis, we have selected three appraisers (A, B, and C) and ten parts that represent typical variation in the diameter output.

→ All appraiser measures the reading of each part 3 times.

→ We use the above data for further calculation.

→ We start by determining the following:

→ The average for each trial for each appraiser

→ Average & Range of each part with each appraiser.

→ The average range for the part

→ The Calculation is mentioned in the below picture.

➨ Putting up these values into the formulas and refer to the below parameters of Gage R&R Study:

Step 5: Interpretation of the Results:

→ We must have variation in the parts and in the appraisers to calculate the above numbers.

→ We have to check the TV to decide that our measurement system is applicable or not?

→ Refer to the bleow mentoined graphical analysis of the data.

→ The number that most people focus on first is the % GRR.

Gage R&R Acceptance Criteria:

→ Gage R&R Acceptance Criteria depend on industry standards and specific company policies, but a common guideline follows AIAG (Automotive Industry Action Group) recommendations:

⏩ Green: GRR % < 10% of TV (Acceptable)

⏩ Yellow: GRR % = 10-30% of TV (Conditionally acceptable based on the application)

⏩ Red: GRR % > 30% of TV (Needs Improvement)   

→ In this example, the measurement system is good because the GRR% value is 3.94% and NDC value is 35.75.

→ Sometimes if the value is higher then we we need to look at the %AV and %EV to get insights into where to start improving the measurement system.

NDC Acceptance Criteria:

→ Also, we need to check the number of distinct categories (ndc) in Gage R&R Study to make the final decision.

→ This is a measure of the number of distinct categories that can be distinguished by the measurement system.

⏩ NDC ≥ 5: Acceptable (Good ability to differentiate parts)

⏩ NDC < 5: Not acceptable. Measurement system is not precise enough and needs improvement.

How to Improve Measurement System?

→ The MSA evaluates the reliability and accuracy of a measurement system.

→ If the MSA results show high variation, corrective actions are needed to improve the system’s precision and effectiveness.

→ Refer to the below meninoted Cause and Effect Diagram For Measurement Variability for your better understanding Measurement System Variation.

→ Now we will discuss improving our measurement system analysis at different levels.

How to Improve Gage Repeatability?

→ We can improve gage repeatability by the following different activities.

→ Calibrate the measurement instrument regularly

→ Use a higher precision gage

→ Use digital measurement tools that will reduce reading errors.

→ If resolution is a problem, use a more precise gage or automated measurement.

→ Control temperature, humidity, and vibration in the measuring environment.

→ Use a stable surface or fixture to hold the part properly.

→ Use fixtures, clamps, or jigs to ensure consistent positioning.

→ Ensure proper gage maintenance

→ Regularly inspect and replace old or faulty equipment.

How to Improve Gage Reproducibility?

→ We can improve gage reproducibility by the following different activities.

→ Train operators on measurement techniques

→ Standardize measurement procedures for all operators.

→ Use standardized measurement procedures

→ Use visual aids, diagrams, and instructions for consistency.

→ Minimize operator influence

→ Use automation or digital measurement tools where possible.

→ Rotate operators in MSA studies

→ Test different operators and identify training gaps.

→ Conduct inter-operator consistency checks.

How to Reduce Part Variation (PV) Issues?

→ Part variation should be large enough to ensure the measurement system can differentiate between different parts.

→ Ensure a representative sample of parts

→ Select parts covering the full range of process variation.

→ Avoid using only similar parts, as it can skew results.

→ Check for process stability issues

→ Conduct a process capability study (Cp, Cpk) to check stability.

→ A small sample size can lead to incorrect conclusions.

→ Increase the number of parts in GRR study

→ A larger sample size (≥10 parts) provides better statistical accuracy.

 Final Takeaways:

→ Regularly calibrate and maintain gages and instruments

→ Train operators and standardize measurement methods.

→ Control environmental conditions (temperature, vibration, etc.).

→ Use automation or digital tools to reduce human error.

→ Ensure proper fixture setups to stabilize measurements.

Benefits:

• Identifies measurement system variability and improves accuracy
• Enhances consistency and reliability
• Reduces waste and rework
• Supports process improvement and decision making
• Ensures compliance with quality standards
• Aids in operator training and standardization
• Helps in gage selection and maintenance

Conclusion:

→ A GRR study is an essential tool for evaluating the reliability and effectiveness of a measurement system.

→ By identifying and quantifying measurement variation caused by equipment and operators, we can ensure data accuracy, process consistency, and decision-making.

→ Also, it helps reduce waste, improve quality, and maintain compliance with industry standards like ISO 9001 and IATF 16949.

→ Finally, a well-executed Gage R&R study leads to better measurement reliability.

→ It leads to enhanced product quality, and improved overall efficiency in manufacturing and process control.