Importance of Performing Gage Repeatability and Reproducibility (GR&R) before actual quality data collection Part 1

From six process inputs (5Ms & 1Es),  only 5 inputs (man, machine, method, material and environment) are contributing to actual process variations and it is inherent to the product.  Measure is actually a process that assess the product quality characteristic. Since measure is process by itself, there is process input variation which will contribute to the process output,  the measurement data.  The observed variation of a process is consist actual process variations and measurement gage variation.

Measurement gage variation comprise of 2 elements ,  repeatability and reproducibility.

Repeatability: closeness of the agreement between the results of successive measurements of the same object carried out under the same conditions of measurement

Can I measure the same thing more than once and get the same answer?

Reproducibility: closeness of the agreement between the results of the measurements of the same object carried out under changed conditions of measurement

Can I change the method of measurement, the observer, the location, the time (next day), and get the same answer?

In order to quantify the variation contributed by measurement gage repeatability and reproducibility we will need to conduct Gage Repeatability and Reproducibility,  GR&R studies.  Per my earlier article on Continuous Improvement Program CIP – 6 sigma  Methodology,  I have mentioned that there are 2 types of quality data, attribute obtain by counting and variable obtain by measuring.  Therefore,  we will also have 2 major types of GR&R studies base on the measurement process of attribute and variable data. 

There are many literature in the internet or textbook which document the method to conduct attribute and variable GR&R studies and I will summarize the methods of conducting GR&R study in this article :-

Before we start any GR&R studies we must ensure that :-

• Variable GR&R study  :-  measurement device must be already calibrated to give accurate data
• Attribute GR&R :-  clear operational definition such as accept or reject criteria must already established before counting or inspection

1. Need to plan for sample size and prepare parts to measure or inspect.  Sample for attribute GR&R should be 3 times more than variable GR&R sample size. 
2. Same operator inspecting measuring the same part multiple times for repeatability studies and record the measurement or inspection data.  Multiple operator who will be conducting the actual assessment (either measurement for variable data or inspection for attribute data) need to measure same part for reproducibility studies.
3. You can either build GR&R calculation template base formula given books in excel spreadsheet or you can use statistical software such as JMP or minitab or Statistica which have this function to find the variation contributed by measurement through GR&R study
4. Variable GR&R calculation is actually base on the percentage how much variation contributed to the total process variation (P/TV) or the total product spec tolerance. (P/PT)

According to Automotive Industry Action Group (AIAG) guideline the acceptance percentage for variable GR&R are per below :-

%  P/TV or P/PT	Judgement	Resulted from	Example
< 10%	Best	Automated inspection or measurement process which do not operator judgement	Automated solder paste height measurement
10-30%	Acceptable	Inspection or measurement which need operator judgement	Measurement of length using Vernier caliper
>30%	Not acceptable	No standardize measurement method Operator not trained Product change over time very fast

• Attribute GR&R is based on agreement of the measurement value between and within operators and operator effectiveness as compare  to standards.  Agreement value is report using a calculated Kappa index between value of -1 to +1,  the higher the value the better. Effectiveness is calculated base on percentage of correct decision over opportunities for decision,  for example I can inspect the part for 100 times and I get it correct only 80 times.  Therefore effectiveness percentage is 80%.Typically the acceptance >80% and best is >90%.

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Do refer to my course for more information on setting up effective inspection system:  

In part 2 of this article,  I will discuss a bit on measurement system analysis (MSA) two important component, precision and accuracy and what is the consequences of not performing GR&R.