Tools

Interpreting the Gage R&R Statistic

The Gage R&R Statistic is usually reported as either a percent of tolerance, or as a percent of the total process variation. Calculate the Gage R&R Statistic easily with our QA-Calibrate gage calibration software, our SPC IV Excel statistical process control software, or our Green Belt XL lean six sigma software.

When expressed as % Tolerance, Gage R&R indicates the relative usefulness of the gage system for determining part acceptance. Tolerance, calculated as the Upper Specification Limit minus the Lower Specification Limit, indicates the amount of variation that is permitted between all parts produced. As gage system error increases relative to the tolerance then the chances increase that:

1. parts whose true value is outside specifications could be measured as being within specifications and subsequently accepted for use, and

2. parts whose true value is within specifications could be measured as being outside specifications and subsequently rejected as unfit for use.

When expressed as % Process Variation, Gage R&R indicates the relative usefulness of the gage system for use in control charting and process capability analysis. The calculation of statistical control limits and the estimate of process capability require a sound estimate of the process variation. Subgroups plotted on a control chart are also subject to measurement system error. As measurement error increases, so does the chance that:

1. Subgroups from a controlled process will be determined to be out of control, and

2. Subgroups from an out of control process will be determined to be from an in-control process.

Typical recommendations for Gage R&R, expressed as ether % Tolerance or % Process Variation, are:

0 to 10%: Acceptable

10-30% Marginal

Over 30%: Unacceptable

The Discrimination Ratio is also useful is determining the suitability of the gage to estimate real process variation.