What is a gage R&R (repeatability and reproducibility) study?


What is a gage R&R study?

A gage R&R study helps you investigate:
  • Whether your measurement system variability is small compared with the process variability.
  • How much variability in the measurement system is caused by differences between operators.
  • Whether your measurement system is capable of discriminating between different parts.

For example, several operators measure the diameter of screws to ensure that they meet specifications. A gage R&R study (Stat > Quality Tools > Gage Study) indicates whether the inspectors are consistent in their measurements of the same part (repeatability) and whether the variation between inspectors is consistent (reproducibility).


Should I use a crossed, nested, or expanded gage R&R study?

Use the gage R&R study that is designed for the type and number of factors that you have.
Crossed gage R&R study
A study in which each operator measures each part. This study is called crossed because the same parts are measured by each operator multiple times. To perform a crossed gage R&R study in Minitab, choose Stat > Quality Tools > Gage Study > Gage Study (Crossed).
Often, you will use a crossed gage R&R study to determine how much of your process variation is due to measurement system variation.
Nested gage R&R study
A study in which only one operator measures each part, usually because the test destroys the part. This study is called nested because one or more factors is nested under another factor and, thus, not crossed with the other factors. To perform a nested gage R&R study in Minitab, choose Stat > Quality Tools > Gage Study > Gage Study (Nested).
Expanded gage R&R study
A study in which one or more of the following conditions exists:
  • More than two factors, usually, operator, gage, and part
  • Fixed or random factors
  • Both crossed and nested factors
  • An unbalanced design
This study is called expanded because you can use it in several types of situations. To perform an expanded gage R&R study in Minitab, choose Stat > Quality Tools > Gage Study > Gage Study (Expanded).

Is my measurement system acceptable?

According to AIAG guidelines, if your measurement system's variation is less than 10% of process's variation, then it is acceptable. To evaluate your process variation, compare the Total Gage R&R contribution in the %StudyVar column (%Tolerance, %Process) in your output with the values in the table.
Percentage of process variation Acceptability
Less than 10% The measurement system is acceptable.
Between 10% and 30% The measurement system is acceptable depending on the application, the cost of the measurement device, cost of repair, or other factors.
Greater than 30% The measurement system is not acceptable and should be improved.

Reference: Automotive Industry Action Group (AIAG) (2010). Measurement Systems Analysis Reference Manual, 4th edition.

Guidelines using variance components

This table contains corresponding guidelines using variance components. To evaluate your variance components, compare the %Contribution column in your output with the values in the table.
Percentage of variance components Acceptability
Less than 1% The measurement system is acceptable.
Between 1% and 9% The measurement system is acceptable depending on the application, the cost of the measurement device, cost of repair, or other factors.
Greater than 9% The measurement system is not acceptable and should be improved.
Important: AIAG also states that the number of distinct categories into which the measurement system divides process output should be greater than or equal to 5.