Intro to Measurement System Analysis (MSA) of Continuous Data – Part 4: Stability

[Miner]

This is the fourth in a series of articles about MSA. The focus of this article will be on measurement stability.

Stability is simply measurement bias throughout an extended period of time. This is where calibration falls short from an MSA perspective. Calibration is a series of snapshots widely spaced in time taken under controlled environmental conditions. A stability study is a series of repeated measurements taken under actual usage conditions. The purpose is to verify that the bias of the gage does not change over time due to environmental conditions or other causes.

A stability study is performed by selecting a measurement standard (ideal) or a master sample part that is midrange of the expected measurement range. Note: This may be enhanced by adding standards/master parts at the low and high ends of the expected measurement range. On a periodic basis, measure the standard 3 – 5 times. The period should be based on knowledge of what may influence the measurement system. For example, if ambient temperature variation is expected to be the major source of variation, make hourly checks throughout the day. If the source of variation is expected to be long term drift, take daily or weekly measurements.

Analyze the data using Xbar/R or Xbar/s control charts (use separate charts if you measured at the low/middle/high ends of the expected measurement range). The subgroups are comprised of the 3 -5 measurements and measure short term repeatability of the measurement device. If the control chart is in a state of statistical control throughout the study period, the gage stability is acceptable. There is no numerical acceptance criterion. If the control chart is out of control, analyze the patterns.

• For example, the influence of temperature would be expected to appear as cyclical trends that coincides with the ambient temperature.
• If the gage operates on plant utilities (e.g., air pressure) abrupt shifts could occur based on plant demand on the utilities (e.g., air pressure).
• Single points out of control could be the result of a gage that is overly sensitive to operator technique.
• Runs could be the result of different measurement methods

A stability study will also provide an estimate of the within operator repeatability of the gage. StdDevrepeatability = Rbar/d*2 (d2 may be used if the number of subgroups is greater than 20).

The next article will be:
Intro to Measurement System Analysis (MSA) of Continuous Data – Part 5: Repeatability & Reproducibility

 

[Marc]

Thank you!

 

[Guest]

Thanks a lot!

 

[Welshwizard]

Understanding that some of these terms are taken from AIAG literature I thought I would comment on the aspect of stability and my approach.

Stability of a measurement process is a fundamental property, in this sense it is no different to any other process that you may choose to study. Clearly, its true that, as you have stated, the reasons why a measurement process is not demonstrated to be stable could be due to many things.
I think the AIAG approach to stability is suggesting that the onus is on the person studying the process to make a judgement as to whether they want to carry out a study for stability, they may for example take the view that the measurement process is going to be sensitive to temperature for example and therefore plan for a stability study.

My preferred approach is to not to possibly take stability for granted from the outset due to the importance of the property to the foundation of the measurement process. If we take the fundamental view that we will use the properties of a process behaviour chart to adjudge whether or not a measurement process is stable by measuring a feature multiple times using one person or machine right from the offset we will quickly come to a conclusions about the stability of our measurement process.

From an engineering perspective, I have learnt far more about the study procedure and the standard operating procedure for using the measurement equipment by performing a stability study than by any other means. Since the objective of any analysis is insight I believe that the role of the stability study, called a consistency study by Dr Wheeler, is very important, for this reason I use it by default.

The stability study stripped bare, i.e. one part measured multiple times by one machine or person, in the is the single most glossed over aspect of any measurement study in my view. Perhaps the consistency aspect is more addressed by AIAG in terms of the Range Chart, by this point more levels have already been added, which is more time and cost and possibly less insight.