Questions about MSA (Measurement System Analysis)

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#1
Hi,
I have some questions about different subjects treated in the MSA manual:

1) In the Average and Range Method at pag. 116, they say that if the analysis is based on the tolerance to evaluate %GRR, ecc it has to be divided by 6.
In the ANOVA method after calculations I have the GRR = 5,15*sigma. To calculate the %GRR I can use the tolerance or there are problems?

2) When I calculate the %GRR for attribute measurement systems with the Analytic Method, can I use both the tolerance or the total process variation? Or can I use only the tolerance?

3) I need to do a flow chart of the controls that an organization need to do to its measurement systems. I know that stability must be done for first, but what about bias, linearity and GR&R? One must be control before the others?

Thank you very much
 
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#3
Re: Questions about MSA

Yes, probably it would help with my third question.
But I have some difficulties to translate some parts, and they say, if I understand good, that I should do GR&R (for statistical stability) and after gage stability. But when I use the control charts Xbar-R for gage stability am I not controlling also statistical stability?
 

Miner

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#4
Re: Questions about MSA

1) In the Average and Range Method at pag. 116, they say that if the analysis is based on the tolerance to evaluate %GRR, ecc it has to be divided by 6.
In the ANOVA method after calculations I have the GRR = 5,15*sigma. To calculate the %GRR I can use the tolerance or there are problems?
First, make sure that you have the latest version. The current version of the MSA manual is the 3rd edition. This version uses 6*sigma, not 5.15*sigma.

The 3rd edition also changed the formulae to simplify them. There are three metrics:

  • P/T Ratio (also called % Tolerance) for gages used as inspection devices. This is (6*sigma)/Tolerance
  • %GRR (also called % Process Variation) for gages used as process control (SPC) or process improvement devices. This is (sigma of MSA)/(sigma of process)
  • ndc (number of discrete categories) for gages used as process control devices

2) When I calculate the %GRR for attribute measurement systems with the Analytic Method, can I use both the tolerance or the total process variation? Or can I use only the tolerance?
I'll leave this question to some others with more experience in attribute studies. My expertise is in variable studies.

3) I need to do a flow chart of the controls that an organization need to do to its measurement systems. I know that stability must be done for first, but what about bias, linearity and GR&R? One must be control before the others?
  1. Bias is another term for calibration, so it should be done first.
  2. Linearity is bias across the entire measurement range. It can be done as part of calibration.
  3. Stability should be next. This is bias over time.
  4. Finally, do the repeatability and reproducibility (R&R).
 
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S

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#5
Re: Questions about MSA

Thank you for your answers.
I still have some doubts, I'm afraid I'm doing confusion.

First, make sure that you have the latest version. The current version of the MSA manual is the 3rd edition. This version uses 6*sigma, not 5.15*sigma.
yes, I have the 3rd Edition.
In Appendix A where they explain the ANOVA Method they use 5,15*sigma.
But I can change it and use 6, so there aren't problems. Right?

  1. Bias is another term for calibration, so it should be done first.
  2. Linearity is bias across the entire measurement range. It can be done as part of calibration.
  3. Stability should be next. This is bias over time.
  4. Finally, do the repeatability and reproducibility (R&R)
About bias, it must be calculate on the production part, right? So when you speak about calibration, you mean the calibration of the instrument for the specific production part?

In the manual is written that a measurement system should be stable before any additional analysis result.
So, I thouht that stability must be done before everything. Probably I don't understand of what stability they are speaking about
 

bobdoering

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#6
Re: Questions about MSA

About bias, it must be calculate on the production part, right? So when you speak about calibration, you mean the calibration of the instrument for the specific production part?

In the manual is written that a measurement system should be stable before any additional analysis result.
So, I thought that stability must be done before everything. Probably I don't understand of what stability they are speaking about
Bias is calibration - it is done on a traceable standard. It is done first to see if the device is measuring correctly at all.

Linearity is part of calibration. It determines if the device is measuring correctly across its measurement region. Again, you need to know that before you can determine stability.

Once you have adequately satisfied bias and linearity, now you can see it they are maintained over time. That is stability. It would be nice to be satisfied about the stability prior to doing the next step - Gage R&R - but you will likely only be able to do short-term stability. After all, at some point you need to use the gage, and its use may affect the stability. :cool:
 
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#7
I'm not able to understand.
In the manual is written that: if the measurement system is stable, tha data can be used to determine the bias of the measurement system.
So why isn't stability the first thing to be control, with control charts?
I know that you are right at 100%, but I want to understand why I'm doing confusion
 
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Miner

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#8
Re: Questions about MSA

yes, I have the 3rd Edition.
In Appendix A where they explain the ANOVA Method they use 5,15*sigma.
But I can change it and use 6, so there aren't problems. Right?
bobdoering answered your second question, so I will focus on the first.

The correct factor to use is 6*sigma. This factor establishes the percentage of measurement variation to be included in the calculation. 5.15*sigma includes an even 99% of the measurement variation while 6*sigma includes 99.73% of the measurement variation.

The MSA 1st and 2nd editions used 5.15*sigma, but the 3rd edition changed to the 6*sigma. I do not have my copy of the MSA manual with me, and I don't remember the context that it was used in Appendix A. However, I do know that 6*sigma is correct.
 

Miner

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#9
I'm not able to understand.
In the manual is written that: if the measurement system is stable, tha data can be used to determine the bias of the measurement system.
So why isn't stability the first thing to be control, with control charts?
I know that you are right at 100%, but I want to understand why I'm doing confusion
The MSA manual describes a method where you perform a stability study on a standard. The difference between Xbar and the standard is bias. The same method can use Rbar to estimate Repeatability. This is mostly a convenience for combining studies like I described for using Linearity to study Bias.

You can make excellent arguments for both sides (Stability then Bias, or Bias then Stability). If you are using gages that have been through multiple calibration cycles without issue, you already have an idea that the gages are stable. I would not be too concerned over which approach you take on this. However, both should be performed before R&R.
 

bobdoering

Stop X-bar/R Madness!!
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#10
I'm not able to understand.
In the manual is written that: if the measurement system is stable, that data can be used to determine the bias of the measurement system.
So why isn't stability the first thing to be control, with control charts?
I know that you are right at 100%, but I want to understand why I'm doing confusion
You may find a short term stability, by trying to determine if the device can repeat in a series of measurements and look for any type of excessive variation or drift. Surely, if you can see that in the short term (and sometimes you do) and the variation is statistically significant, then you need to get the device fixed or get a better one. I suppose it is hard to describe unless you have a bad gage in front of you to show you the problem.

But short term instability really is not as common as long term instability. If calibration is done correctly, you can actually track it from the calibration data rather than doing a special test. The key is to use the same standards when you calibrate, and track before and after values. :cool:
 
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