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
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
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
Use the control chart to check the interval
Use the control chart at regular intervals
(e.g. weekly) to make sure the device remains
If a (previously) stable device becomes
unstable, you will have exceeded the safe
interval of calibration or repair.