Measurement System Stability

Stability helps evaluate how a measurement device performs over time.

• Stability looks at whether the bias changes over time. If the bias does change, the device is less repeatable over time.

• Stability studies also help determine the proper increment between calibration and repair intervals.

Stability can be analyzed using an X-Bar & R or IX & MR (individuals and moving range) control chart.

A reliable, stable standard is needed to begin a stability study.

• The same standard will be used during the entire study.

• The standard must not degrade over time, wear as it is measured, or have a short shelf life.

Control chart method of evaluating stability:

• Measure the sample 3 to 5 times. Calculate X-Bar & R and plot on a control chart.

• Repeat daily for 20 days.

• After 20 sets of measurements, calculate the control limits and add them to the chart.

• Are there any out-of-control conditions indicating instability? If so, the device is not stable.

Use the control chart to check the interval for calibration.

• Use the control chart at regular intervals (e.g. weekly) to make sure the device remains stable.

• If a (previously) stable device becomes unstable, you will have exceeded the safe interval of calibration or repair.