Continuous Gage R&R – A Step by Step Approach

In this post, we will look at the step by step approach to perform Continuous Gage R&R. It is also referred to as Gage R&R. We will also explore the results of Continuous Gage R&R and how to interpret these results.

In my earlier post, we looked at what Measurement System Variation means, how it can creep into your process data and what steps you should take to ensure that your data is free of such variation. We also looked at an overview of Measurement System Analysis (MSA). MSA is done to ensure that your measurement system is robust, precise and accurate. Please click on the links to read these posts (opens in new tab). We will build on the examples we discussed in these posts.

When you have Discrete data set, you perform Attribute Gage R&R. In our example, the Acceptability data set of the wooden planks is discrete. Hence, we used this data to perform Attribute Gage R&R. Click here to read the step by step approach for Attribute Gage R&R (opens in new tab).

However, the length of the wooden plank is continuous data. If your data set consists of continuous data, you need to perform Continuous Gage R&R. Please remember, it is extremely important to do the correct MSA depending on the type of your data. Else, these tests will be of no use.

Key Assumptions

The Key assumptions before we do Continuous Gage R&R stays the same as with Attribute Gage R&R. You should have completed all the pre-work as discussed in Measurement System Analysis (MSA) overview post. Which means, you have selected the correct parts for performing MSA, you have numbered the parts, you have identified the operators for this test, you have selected the right measuring equipment and you have the data collection template ready. Below is a simple data collection template that we will use.

We will use the 10 selected parts, 3 operators will measure the length of these 10 wooden planks, 2 times each. And will store the results in the below data collection template.

Steps to do Continuous Gage R&R

Step 1 : Operator A measures the length of each of the 10 selected wooden planks and records it in the first column – Operator A – Trial 1.

Step 2 : Give these 10 parts to operator B. He measure the length of each of these planks and record it in the 3rd column – Operator B – Trial 1.

Step 3 : Operator C repeats the same exercise and records the length of the wooden planks in fifth column – Operator C – trial 1.

Once the above 3 steps are done, your data collection template will look somewhat similar to this;

Step 4 : Repeat step 1 to 3 in the same order with the same parts and same operators for trial 2.

Ensure that you do not let the operators see each others results as well as their own trial 1 results while doing this exercise as it may influence their decisions. Your data collection template now looks like below;

Continuous Gage R&R – Data Structure and Test

Now that you have the required data with you, its time to do the test. You can use any statistical analysis software, or a macro enabled MSA template that you can easily download from the internet, to do the test. I will use MiniTab, one of the preferred statistical analysis software for Lean Six Sigma practitioners. However, the results and the takeaways will stay the same irrespective of what software / template you use.

You need to structure the data in a specific format to use it in MiniTab. You need to put this whole data into 3 columns. The first column is for Part numbers, second for Operator names and third for results (lengths). It will look similar to the image below. Copy this data set into Minitab worksheet.

The MiniTab test to do Continuous Gage R&R is called Gage R&R Study (Crossed). The path for this test in MiniTab is as shown below – 

Stat > Quality Tools > Gage Study > Gage R&R Study (Crossed)

Gage R&R Study Command Box

Once you click on the above test, the following command box appears.

The command box asks for 3 inputs – part numbers, Operators and measurement data (Length). Select the correct data column for each of these 3 fields as shown in the below image. Keep the selection of method of analysis as ANOVA.

Once you have selected the correct input, click on “Options” button. The options command box will appear. The options box asks for multiple inputs. Do not change any pre-populated values for study variation and alpha. Select the first radio button for process tolerance and input the lower and upper specification limits as shown in the below image.

Remember from our discussion in Measurement System Analysis overview post, our customer will accept planks of length between 149 cm and 151 cm. These are our specification limits.

Click on Ok in both the command boxes to run the test.

Continuous Gage R&R – Results

You get MiniTab test results in 2 forms, a textual output and a graphical output. Below are both the results;

As established in all my previous posts, do not get overwhelmed or confused by all the numbers and statistics that MiniTab throws at you in the results. You need to know what exactly you need to look at and how to interpret it.

For Continuous Gage R&R, there are 4 important statistics that you need to look at. Total Gage R&R % Tolerance, R&R % Contribution, Number of distinct categories and R&R % Study Variation. These values can be found in the textual output as highlighted in the below image.

Total Gage R&R % Tolerance

R&R % Tolerance value should be less than 30% for the Gage to be acceptable. If this value is greater than 30%, the Gage is unacceptable. You will need to revisit your measurement system and fix any problems that exists in the measurement system. You should also see if there is a better measuring system / instrument available in the market and decide is the cost of such an improved Gage is justifiable. Additionally, you can also check if all your operators understands how to use the measuring instruments.

By the way, do check out the Certified Lean Six Sigma Black Belt Handbook – it is one of the most essential guide for anyone trying to get certified as LSS Black belt or in general wants to understand LSS and improve processes. – check it out here.

R&R % Contribution

% contribution for total Gage should be very small compared to the % contribution of Part tot Part variation. An acceptable R&R % contribution is below 8%. Below 2% is optimum. This means that majority of variation in your data is because of actual process variation and not because of the measurement system. If the contribution of total total Gage R&R % contribution is higher than 8%, the measurement system is unacceptable and needs to be corrected.

Related Post : What are measures of Variation?

Number of Distinct Categories

Number of distinct categories refers to the ability of the measuring equipment to distinguish between the readings of different parts. It is the ability of the equipment to differentiate the parts based on how precisely the readings can be recorded. If your measuring tape cannot differentiate between 149.5 and 149.6, it wont be an effective gage, right?

Higher the number of distinct categories, better your Gage. The acceptable number of distinct categories is 4 and above. If you have less than 4 distinct categories, the Gage is not acceptable and you need to revisit the measurement system.

This is all about Continuous Gage R&R. Thank you for reading. Please feel free to let me know if you have any particular questions regarding this post or Gage in the comment section below and I would be happy to answer.

Sachin Naik

Passionate about improving processes and systems | Lean Six Sigma practitioner, trainer and coach for 14+ years consulting giant corporations and fortune 500 companies on Operational Excellence | Start-up enthusiast | Change Management and Design Thinking student | Love to ride and drive

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