# Stability

Stability is the total variation in the measurements obtained with a measurement system on the same master or parts when measuring a single characteristic over an extended time period.

Obtain a sample and establish its reference value(s). If one is not available, select a production part that falls in the mid-range of the production measurements and designate it as the master sample for stability analysis. The known reference value is not required for tracking measurement system stability.

On a periodic basis (hourly, daily, weekly), measure the master sample three to five times. The sample size and frequency should be based on knowledge of the measurement system. Factors could include how often recalibration or repair has been required, how frequently the measurement system is used, and how stressful the operating conditions are. The readings should be taken at different times to represent when the measurement system is actually being used. This is account for warm-up, ambient or other factors that may change during the day.

Plot the data on an XBar R or XMR control chart.

Establish control limits and evaluate for out of control or unstable conditions.

Compute the standard deviation for the measurements and compare it with that of the process to determine if the measurement system reliability is suitable for the application.

Study to find the Stability of the measurement system is divided into three parts - Stability Mapping, Stability Measurement and Stability Data Analysis.