GRR Study in MSA

What is a Gauge R&R study in MSA?

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

→ Gauge R&R study or “Gauge Repeatability and Reproducibility 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.
Gage R&R 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.
Gage R&R 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.
Gage R&R 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:
Gage R&R study in MSA
⇒ Some other values of K3 for different numbers of parts:

Gage R&R study in MSA

➤ [5] Total Variation (TV) :

      ➧ This is the total variation from the study. It is determined by the following equation:
Gage R&R study in MSA

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.


Gage R&R study in MSA

👉 Click here: GRR Study Excel Template Download

      ➧ 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.

Gage R&R study in MSA

👉 Click here: GRR Study Excel Template Download

→ Determine the average range for the three appraisers.

Gage R&R study in MSA

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

Gage R&R study in MSA

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

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

Gage R&R study in MSA

      ➧ 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:

Gage R&R study in MSA

👉 Click here: GRR Study Excel Template Download

 ➤ 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).


Gage R&R study in MSA

👉 Click here: GRR Study Excel Template Download

      ➧ 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). 

      ➧ 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

      ➧ Brainstorm with the team for improvement solutions.
      ➧ Determine 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.



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