R&R time: General Industry Coverage

True or false: Measuring parts and collecting data, unlike other processes, are not subject to variations.

This statement is, of course, false. Variation is inherent to any system, and the measurement and data-collection processes are no exception.

All images courtesy PQ Systems

A digital caliper and a digital micrometer.

A perfect measurement system does not exist; every system introduces some amount of variation. An excessive amount of variation in the measurement system may lead to false signals on control charts and untrustworthy recorded measurements that render the process of analyzing data with statistical-process-control tools futile.

Quality managers strive to reduce or limit variation in their measurement systems. In addition to using operational definitions to ensure measurement consistency, companies should periodically perform repeatability and reproducibility (R&R) studies on their gages and regularly recalibrate them.

Most part manufacturers understand the importance of regular calibration to ensure their gages and other measurement devices are within acceptable tolerances. But while calibration assesses the accuracy of the measurement device, it doesn’t guarantee overall measurement accuracy. Outside influences that affect measurement systems are always present. For example, even an extremely precise and accurate gage, if used by a poorly trained operator, may generate faulty results.

The Two ‘Rs’

A comprehensive plan to reduce measurement variation includes conducting gage R&R studies. While calibration assesses only gage accuracy, a GR&R study can identify many areas of variation. Most notably, a completed study will yield a breakdown of the contributing sources of equipment and appraiser variation. These two elements, combined, yield an R&R percentage.

The first R, repeatability, reflects measurement error when all factors are kept constant: the same appraiser, gage, measurement procedure, and part or sample. This part of the study determines what variation can be attributed to the equipment. Reproducibility, the second R, compares the measurements of all appraisers (typically the same technicians that use the gages) to determine if there is a significant amount of variation among them.

Regularly scheduled GR&R studies can be incorporated into an organization’s quality system. The first study can serve as a baseline to determine how well the measurement system is performing. With the initial study recorded, quality managers can then investigate the main sources of variation. With that knowledge, they can take appropriate steps to correct and reduce measurement system variation.

GR&R studies usually open a dialog between the gage users and their supervisors. This discussion can lead to improved measurement procedures and even operator retraining, if necessary.