What is a Gauge R&R study in MSA? | GRR Study | Variable MSA Study

What is a Gauge R&R study in MSA? | GRR Study | Variable MSA Study

1) What is a Gauge R&R study in MSA?

→ Gauge R&R study or “Gauge Repeatability and Reproducibility Study (GRR Study)” is a term which is commonly used in MSA.

→ To calculate the GR&R %, following five contributors are to be calculated:

  1. Equipment Variation (EV)
  2. Appraiser Variation (AV)
  3. Gauge Repeatability & Reproducibility (GRR)
  4. Part Variation (PV)
  5. Total Variation (TV)



➤ [1] Equipment Variation (EV) in GRR study in MSA:

      ➧ This is the "within appraiser" variation. 
      ➧ It measures the variation one appraiser has when measuring the same part 
         (and the same characteristic) using the same gauge more than one time.
      ➧ The calculation is given below.
Equipment Variation (EV) in GRR study in MSA

⇒ 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.



➤ [2] Appraiser Variation (AV) in GRR study in MSA:

      ➧ This is the "between appraisers" variation.
      ➧ It is the variation in the average of the measurements made by the different appraisers when measuring the same characteristic on the same part.
      ➧ The calculation is given below.
Appraiser Variation (AV) in GRR study in MSA

 Where K2 is a constant that depends on the number of appraisers.
 For 2 appraisers, K2 is 0.7071.
 For 3 appraisers, K2 is 0.5231 

➤ [3] Gauge Repeatability & Reproducibility (GRR) in GRR study in MSA:

      ➧ GRR is the combination of AV and EV and the calculation is given below.
Gauge Repeatability & Reproducibility (GRR) in GRR study in MSA

➤ [4] Part Variation (PV) :

      ➧ The part variation is determined by multiplying the range of the part averages (Rp) by a constant K3.
      ➧ K3 depends on the number of parts.
      ➧ The part variation is then given by:
Part Variation (PV)

⇒ Some other values of K3 for different numbers of parts:

values of K3 in MSA



➤ [5] Total Variation (TV) :

      ➧ This is the total variation from the study. It is determined by the following equation:
Total Variation (TV)

2) Steps for Gauge Repeatability and Reproducibility (GR&R) Study in MSA:

  1. Select 10 parts that represent the full range of long-term process variation.
  2. Identify the appraisers (inspectors or part checker).
  3. If appropriate, calibrate the gauge or verify that the last calibration date is valid.
  4. Open the Gauge R&R worksheet to record data.
  5. Have each appraiser assess each part 3 times (trials – first in order, second in reverse order, third random).
  6. Input data into the Gauge R&R worksheet.
  7. Enter the number of operators, trials, samples and specification limits
  8. Analyze data in the Gauge R&R worksheet.
  9. Assess MSA trust level.
  10. Take actions for improvement if necessary.




➤ Step 1: Determine the number of parts, the number of appraisers to use and the number of trials - GRR study in MSA

      ➧ There are several issues that must be considered when planning a gauge R&R study.
      ➧ The first is the number of appraisers and the number of parts to use.
      ➧ The number of parts (n) must be greater than or equal to 5.
      ➧ The number of appraisers (k) must be greater than 2.
      ➧ The number of trials (r) must be greater than or equal to two.
      ➧ This represents how often each appraiser will measure a part.
      ➧ In addition, the n*k should be greater than 15.
      ➧ This gives more confidence in the results.
      ➧ If possible, include all the appraisers who operate the gauge in the study.


➤ Step 2: Select the parts for the study - GRR study in MSA

      ➧ The next step is selecting the parts to include in the study.
      ➧ The parts should be selected to reflect the range of variation in the process. 
      ➧ In other words, don't just take 10 parts off the line right in a row.
      ➧ You need to select the parts so they reflect the variation seen in 
         the manufacturing process.


➤ Step 3: Label the parts from 1 to n and designate the appraisers A, B, etc. - GRR study in MSA



➤ Step 4: Conduct the Measurements - GRR study in MSA

      ➧ The parts must be run in random order. 
      ➧ Start with appraiser A. Appraiser A measures the parts in random order.
      ➧ The results are recorded.
      ➧ This process continues for each appraiser without the appraisers being able to see 
          the results from other appraisers.
      ➧ This cycle is continued until you have completed all trials.
      ➧ Be sure that an appraiser cannot see his/her results from previous trials.




 ➤ Step 5: Analyzing the Results - GRR study in MSA

      ➧ You select three appraisers (A, B, and C). 
      ➧ You select five parts that represent typical variation in the length output. 
      ➧ You have each appraiser measure each part three times. 
      ➧ The measurement results are given below.


Analyzing the Results - GRR study in MSA

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      ➧ You use the above results to perform the gauge R&R calculations.
      ➧ You start by determining the following:
      ➧ The average for each trial for each appraiser
      ➧ The average and range for each part for each appraiser
      ➧ The overall average and average range for each appraiser
      ➧ The overall average and the average range for the part
      ➧ These calculations are shown in the table below.

Calculation of recorded data

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→ Determine the average range for the three appraisers.

Determine the average range for the three appraisers

 Then, determine the difference between the maximum appraiser average and 
     the minimum appraiser average. 

determine the difference

      ➧ A has the maximum average (3.157). C has a minimum average (2.695).
      ➧ Thus,  the difference is (3.157) - (2.695) = 0.462

→ Next, determine the range of the part averages (Rp). 

determine the range of the part averages (Rp)

      ➧ The largest part average is for Part 3 (4.10). 
      ➧ The smallest part average is for Part 5 (1.94).
      ➧ So, Rp = (4.10) - (1.94) = 2.16



→ Putting up these values for all the 5 contributors:

Putting up these values for all the 5 contributors

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 Step 6: Interpreting the results - GRR study in MSA

      ➧ Remember, a gauge R&R study is a study in variation.
      ➧ You must have variation in the parts and in the appraisers to calculate 
         the above numbers.
      ➧ To determine if the measurement system is adequate, you must compare
         the results to Total Variation (TV).


Calculation of TV

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      ➧ The number that most people focus on first is the % GRR
      ➧ The following guidelines can be used to determine if the measurement system
          is acceptable, conditionally acceptable or not acceptable.

      ➧ Green: < 10% of TV (Measurement system is acceptable)
      ➧ Yellow: 10-30% of TV (Measurement system may be acceptable
                                               based on the application)
      ➧ Red: > 30% of TV (Measurement system needs improvement)      

      ➧ In this example, the measurement system needs improvement since 
         %GRR is greater than 30%.
      ➧ Sometimes you can look at the %AV and %EV to get insights into where to start
          improving the measurement system.
      ➧ But the study indicates that the measurement system must be improved.




➤ You can also determine the number of distinct categories (ndc) in Gauge R&R Study

      ➧ This is a measure of the number of distinct categories that can be distinguished by the measurement system.
      ➧ It is similar to looking at how many possible values there are on a range control chart.
      ➧ The calculation is:
          ndc = 1.41(PV/GRR) = 1.41(0.872/0.319) = 3.8
      ➧ The integer value of ndc should be greater than or equal to 5.
      ➧ In this case, it is 3. 
      ➧ Again, this is an indication that the measurement system needs improvement.



➤ If repeatability is large compared to reproducibility, then the reasons may be:

    1. The instrument needs maintenance;
    2. Other reason may be the gauge should be redesigned to be more rigid;
    3. The clamping or location for gauging needs to be improved;
    4. There is excessive part variation.

➤ If reproducibility is large compared to repeatability, then possible causes may be:

    1. The appraiser needs to be better trained in how to use and read the gauge instrument.
    2. Calibrations on the gauge dial are not clear
    3. A fixture of some sort may be needed to help the appraiser use the gauge more consistently.

➤ Some improvement points in Gauge R&R study in MSA

      ➧ Brainstorm with the team for improvement solutions.
      ➧ Determine the best “practical solution” (may require some experimentation).
      ➧ Pilot the best solution
      ➧ Implement the best solution – train employees.
      ➧ Re-run the study to verify the improvement.