MSA: Graphical Interpretation (Mean and Range Chart)

“Design is intelligence made visible.” – Alina Wheeler

Introduction

Sometimes we make decisions based on the data. Sometimes we do it based on the hunch feeling. Many times, we do not take any decision, although this is also a decision. But in all cases, whatever decision we take makes an impact on our lives. The more logical, informed and practical decisions we take, the better will be its outcome.

Content: MSA: Graphical Interpretation of Mean and Range

1.?????? What is the Mean Chart?

2.????? What is a Range Chart?

3.????? What is the purpose of the Mean and Range Chart?

4.????? How to interpret it?

5. Conclusion

Objective

Measurement Systems Analysis, as part of the Statistical Process Control practices, has been widely used in many industries, especially those related to the automotive sector.

When the measurement of any component is conducted, apart from the process variation, there is also the contribution of error from the measurement system. As per the MSA manual 4th edition, graphical interpretation of the mean and range chart helps the user to understand, whether the sample collection was right and if the variation due to the measurement system is under control or not.

Once you go through the article, you will understand the meaning of the Mean and Range chart, what is the purpose of these graphs, how to read these graphs and what action needs to be taken.

Read More: https://bit.ly/VariableAttributeStudy

Definition: MSA: AIAG Manual 4th Edition

Repeatability: Variations in the measurements obtained with one measuring instrument when used several times by an appraiser while measuring the identical characteristic on the same part.

Reproducibility: Variation in the average of the measurements made by different appraisers using the same gauge when measuring a characteristic on a part.

Bias: The difference between the observed average of measurements and the reference value.

Linearity: The change in bias over the normal operating range. A systematic error component of the measurement system.

Stability (Drift): The total variation in the measurement obtained with the measurement system on the same master or parts when measuring a single characteristic over an extended time period. Stability is the change in bias over time.?

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Detailed Information

The purpose of Measurement System Analysis is to qualify a measurement system for use by quantifying its accuracy, precision, and stability.

The key purpose of conducting the study is to understand that,

-???????? Whether our measurement system variability is small compared with the

process variability.

-???????? Whether our measurement system is capable of distinguishing between

different parts.

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In any measurement system, there can be 5 different errors. They are,

-???????? Bias

-???????? Stability

-???????? Linearity

-???????? Repeatability

-???????? Reproducibility

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Graphical Interpretation:

In the MSA study, we plot two types of graphs (Mean and Range). Although the calculation of Parts Variation (PV), ndc and R&R% provides a lot of information and understanding about the contribution of measurement error in the total process variation, the understanding of the Mean and Range chart provides more clarity and understanding of the measurement system error.

A systematic screening of the data for apparent special causes of variations by using graphical tools should precede any other statistical analysis.

Read More: https://bit.ly/BiasLinearity

Range Chart:

The Range chart should be analysed First. All the points should fall within the control limits (between LCL and UCL).

Any point outside the upper control limit on the range chart or any unusual patterns of variation (such as all of the range values for one appraiser being below the average range) is a sign that the measurement system is out of control. It should be investigated and improved.

If one appraiser is out of control, the method used differs from others. Similarly, if all the appraisers have some out-of-the-control ranges, it means, the measurement system is sensitive to the appraiser's technique and needs improvement to obtain useful data. Example: Measurement of rubber hardness by Hardness Tester.

Read More: https://bit.ly/MSASWIPE

Mean Chart:

The Average chart indicates the ‘Usability’ of the measurement system. The area within the control limits represents the measurement sensitivity (NOISE). With the Averages (X-bar) chart, we “want” to see points outside of the control limits.

• This is different from our goal with a standard averages chart. With standard average charts, we want all points to be in control. We do not want that here.
• A Mean and Range control chart on the measurement system analysis results must have range values for each part/sample and operator in control. But unless the majority of the values on the X-bar chart are outside of the control limits, then the measurement system is not adequate.
• Unless more than half the averages are outside of the control limits, the measurement system has considerably more variation than the total part, or process, variation so it cannot adequately discriminate between good and bad parts.
• The X-bar chart should have roughly the same pattern for all of the appraisers. If the patterns of the appraisers are very different, there might be some differences in their techniques that should be investigated.
• If less than half falls outside the control limits then either the measurement system lacks adequate resolution or the samples do not represent the expected process variation.

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Conclusion:

The purpose of the MSA study is to identify the possible error emanating from the measurement system. Either the error should be within target or the user should have a justified reason to have more measurement error. Whatever decisions are being taken; it should be in context to the measurement system. In short, the smaller the uncertainty, the more reliable the data.

Read More: https://bit.ly/CommonSpecialCause

Industry Challenges:

1. How many users of the MSA study have clarity regarding the graphical interpretation of the mean and range chart?

2. Even if the graphs reflect the fulfilment of the requirement, do we

understand, why it happened?

3. How often do the users take action when the mean and range chart does not

meet the requirement?

References:

MSA AIAG Manual 4th Edition

Industry Experts

This is the 209th article of this Quality Management series. Every weekend, you will find useful information that will make your Management System journey Productive. Please share it with your colleagues too.

In the words of Albert Einstein, “The important thing is never to stop questioning.” I invite you to ask anything about the above subject. Questions and answers are the lifeblood of learning, and we are all learning. I will answer all questions to the best of my ability and promise to keep personal information confidential.

Your genuine feedback and response are extremely valuable. Please suggest topics for the coming weeks.