Gage Stability

Gage Stability is a measure of whether the measurement system is subject to wear or other time-based deterioration. It is evaluated by taking repeat bias samples of the same part, over the course of time. A control chart is used to assess whether the measurement system is in statistical control.

Typically, an X-Bar control chart is used. Each subgroup on the control chart represents the average of repeat measurements of the same sample piece. This same sample piece is used for each subgroup. The data values can be used as measured, or converted to bias estimates (by subtracting from the reference standard value).

Out of control or Run Test violations on the Stability control charts may be used to improve the measurement system. If the out of control condition is verified as a shift in the measurement process, its occurrence can be used to indicate the need for Calibration. Calibrating the equipment at times other than this may lead to Tampering, which actually increases the measurement variability.

It is important to use knowledge of the measurement system when considering the timing of the subgroups. Warm up periods, for example, should not be included in the sampling if process or product measurements do not occur until after conclusion of the warm-up period. Vary the time of day that subgroups are taken if temperature, humidity, or other factors that vary throughout the day could influence the measurements.

When the measurement system is not stable, estimates such as R&R and Linearity are not useful for predictions of the measurement system future performance.

Learn more about the Quality Improvement principles and tools for process excellence in Six Sigma Demystified (2011, McGraw-Hill) by Paul Keller, or his online Green Belt certification course ($499).