# Difference between Stability and Consistency

## Is there any difference in stability and consistency?

• ### No

Votes: 0 0.0%

• Total voters
6
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#### Sean Kelley

We are currently getting ready to perform a gauge r&r study. We are also attempting to fulfill automotive requirements of Accuracy, Bias, Stability, Linearity and Consistency. We have not been successful at finding a difference in Stability and Consistency. Any suggestions when first performing these studies would also be appreciated.

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#### Graeme

Sean Kelley said:
We have not been successful at finding a difference in Stability and Consistency. Any suggestions ... would also be appreciated.

Sean,

Here are a couple of definitions ...

Stability: the ability of a measuring instrument's metrological characteristics to remain constant over time. (paraphrased from the ISO International guide of basic and general terms in metrology, 1993; item 5.14.) From this, Stability is a property of an individual measuring instument -- its variation over time.

Consistency: a measure of the agreement between two or more sets of measurements of a quantity made using a given method; but when made at different times, with different operators and possibly with different instruments. (Interpreted from page 173 of An introduction to error analysis second edition, John R. Taylor, 1982.) Consistency is a property of a group of measurement results, and how closely they agree with each other.

Considering that one has to do with an individual instrument and the other deals with a group of results of a measurmenet process, then Yes, they are different. Stability of an instrument may affect the consistency of a group of measurements, but the reverse can never happen. A group of measurements may be inconsistent even if all of the instruments are stable, because there are other influences in the measurement process.

Does this help?

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A

#### Atul Khandekar

Stability & Consistency

Sean Kelley said:
We have not been successful at finding a difference in Stability and Consistency.
Stability is the measure of drift in the measurements taken over a period of time. It is the change in bias over time. Consistency is the measure of change in variation over time. So, consistency is related to repeatability over time.

To monitor both stability and consistency, you should do the Stability Study as prescribed in the AIAG MSA manual. Plot XBar and R charts. The drift in the measurement is seen on the XBar chart, so if the XBar chart shows out-of-control then measurement system stability is in question.

Repeatability can be monitored on the Range chart. If range chart is under control, the measurement system is consistent. The standard deviation can be used as a rough estimate of repeatability.

-Atul.

#### Jim Wynne

Leader
Admin
Sean Kelley said:
We are currently getting ready to perform a gauge r&r study. We are also attempting to fulfill automotive requirements of Accuracy, Bias, Stability, Linearity and Consistency. We have not been successful at finding a difference in Stability and Consistency. Any suggestions when first performing these studies would also be appreciated.

Atul and Graeme have provided excellent answers. The problem lies in semantics, and the fact that "stability" is a type of consistency in the broad scheme of things, but it's necessary to provide specialized definitions in order to maintain the important distinction that Graeme and Atul have described. It's important to remember that sometimes words take on different meanings when they're applied to specialized circumstances; it's a form of jargon.

H

#### H. Majhenich

H. Majhenich

Like a square can be a rectangle, but a rectangle can't be a square?

#### Jim Wynne

Leader
Admin
H. Majhenich said:
Like a square can be a rectangle, but a rectangle can't be a square?

It all depends on how your customer defines "square."

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