Gage R&R Overview
 

Gage repeatability and reproducibility studies determine how much of your observed process variation is due to measurement system variation. Minitab allows you to perform either crossed, nested or general Gage R&R studies.

·    Use Gage R&R Study (Crossed) when each part is measured multiple times by each operator. You must have a balanced design with random factors.

·    Use Gage R&R Study (Nested) when each part is measured by only one operator, such as in destructive testing. In destructive testing, the measured characteristic is different after the measurement process than it was at the beginning. Crash testing is an example of destructive testing. You must have a balanced design with random factors.

·    Use Gage R&R Study (Expanded) when you need to include more factors than Operator and Part, have fixed factors, a mixture of crossed and nested factors, or an unbalanced design. You can have a maximum of eight factors in addition to Part and Operator. All factors can be fixed or random, and crossed or nested. The design can be balanced or unbalanced.

To help setup your worksheet for the Gage R&R studies, use Create Gage R&R Study Worksheet.

Assessing Repeatability and Reproducibility

Minitab provides two methods for assessing repeatability and reproducibility: X and R, and ANOVA. The X and R method breaks down the overall variation into three categories: part-to-part, repeatability, and reproducibility. The ANOVA method goes one step further and breaks down reproducibility into its operator, and operator-by-part, components.

The ANOVA method is more accurate than the X and R method, in part, because it considers the operator by part interaction [8, 9]. Gage R&R Study (Crossed) allows you to choose between the X and R method and the ANOVA method. Gage R&R Study (Nested) and (Expanded) uses the ANOVA method only.

Choosing Crossed, Nested, or Expanded

If you need to use destructive testing, you must be able to assume that all parts within a single batch are identical enough to claim that they are the same part. If you are unable to make that assumption then part-to-part variation within a batch will mask the measurement system variation.

If you can make that assumption, then choosing between a crossed or nested Gage R&R Study for destructive testing depends on how your measurement process is set up. If all operators measure parts from each batch, then use Gage R&R Study (Crossed) or (Expanded). If each batch is only measured by a single operator, then you use Gage R&R Study (Nested) or (Expanded). In fact, whenever operators measure unique parts, you have a nested design.

If you need to include more factors than Operator and Part, have fixed factors, a mixture of crossed and nested factors, or an unbalanced design, you must use Gage R&R (Expanded).