Hello people,

Just want to know about the difference between MSA and Measurement Capability Analysis? Hope to hear your reply.

Thanks!
Errol

Hello people,

Just want to know about the difference between MSA and Measurement Capability Analysis? Hope to hear your reply.

Thanks!
Errol
Measurement capability analysis and MSA are one and the same.

Just want to know about the difference between MSA and Measurement Capability Analysis? Hope to hear your reply.Welcome to the Cove, Errol! :bigwave:

Yes, they are basically the same. The whole exercise of MSA helps you quantify the two types of variation (position variation - bias, linearity and width variation-GRR) for measuring a particular characteristic.

Various indexes can be computed.

A. Based on a Bias/Repeatability Study

(Ref: My old notes based on the booklet "Capability of Measuring Instruments": Robert Bosch)
Take repeated measurements (25-50) on a production master, calculate average and Repeatability StDev.

The two indexes calculated are:
1. Cgm = (0.2 * Tolerance) / (6 * Rpt_Stdev)
2. Cgmk = Least of
( RefValue + (0.1 * Tolerance) - XBar) / (3 * Rpt_StDev)
-OR-
( XBar - (RefValue - (0.1 * Tolerance)) ) / (3 * Rpt_StDev)

Both should be >= 1.33; and bias (XBar-RefValue) not significant.

B. Based on GRR, typically two indexes are recommended:

1. Measurement Capability Index (MCI_I) based on Process Variation is defined as:
MCI_I = 100 (Sigma_GRR/Sigma_process)
Sigma_Process is calculated from ongoing SPC studies: RBar/d2

2. The other index, (MCI_II) is based on tolerance:
MCI_II = 100 (5.15 * Sigma_GRR/ Tolerance )
In place of 5.15 you can also use 6 to cover 99.73% spread.

Acceptance criteria for both these are:
<=20% :Good
>20 - <=30% :Marginal
>30% :Unacceptable

Thank you so much for your replies.It's help me a lot :thanx:

Found that the Constant for GR&R shown on the example in page 114 of MSA 3rd edition is not same as the sample shown in SPC manual.

i.e Trial (3) k1 = 0.5908 in MSA manual but in SPC manual
Trial (3) K1 = 3.05 .

Is there a reason for this. Am i missing something big ?

Need help !!! Spiderman, Superman , Super-Consultant..

Thanks and Regards.

The constants changed between the 2nd and 3rd edition of MSA.
I am afraid that I cannot explain the statistics but the effect is supposed to be minor.

Thanks for the reply howard.

The new constant stated in the MSA 3rd edition. Which page is this. Sorry to ask but im really confused and cant find it.

Thanks and regards.

Thanks for the reply howard.

The new constant stated in the MSA 3rd edition. Which page is this. Sorry to ask but im really confused and cant find it.

Thanks and regards.
You've already found it--it's the form you referred to in an earlier post, on page 114 of the MSA manual.

Wow. and i thought that's an example only. And there'll be the actual constant in the manual somewhere in a nice table format.

Confused and dumb-founded ...

Thanks and regards.

What ever happened to the old "10:1" rule for measurement systems? Seems like MSA and other analyses are just rehashing the idea that the measurement system should not take up more than 10% of your total process variation. Where did I lose it? :D

What ever happened to the old "10:1" rule for measurement systems? Seems like MSA and other analyses are just rehashing the idea that the measurement system should not take up more than 10% of your total process variation. Where did I lose it? :D

The 10:1 rule and GR&R results are not directly related. The former says that the device used should be 10x as sensitive as the measurement required. In other words, if you have need to measure to .001", the device should be capable of .0001" discrimination. The 10% GR&R target takes other factors into consideration, such as operator error and part-to-part variation. Whether or not 10% is appropriate is dependent on the criticality of the measurements and the amount of tolerance available. If the process is already using 95% of the tolerance, 10% might be too much. On the other hand, if the process is using only 10% of the tolerance, more gage error is tolerable.

By MSA analysis

We have decided that

Our system is capable of either checking or analysing the particular data.

Through its not giving exat value like cpk or cp but still MSA & Measurement

Capability Analysis is the same thing.

Welcome to the Cove, Errol! :bigwave:

Yes, they are basically the same. The whole exercise of MSA helps you quantify the two types of variation (position variation - bias, linearity and width variation-GRR) for measuring a particular characteristic.

Various indexes can be computed.

A. Based on a Bias/Repeatability Study

(Ref: My old notes based on the booklet "Capability of Measuring Instruments": Robert Bosch)
Take repeated measurements (25-50) on a production master, calculate average and Repeatability StDev.

The two indexes calculated are:
1. Cgm = (0.2 * Tolerance) / (6 * Rpt_Stdev)
2. Cgmk = Least of
( RefValue + (0.1 * Tolerance) - XBar) / (3 * Rpt_StDev)
-OR-
( XBar - (RefValue - (0.1 * Tolerance)) ) / (3 * Rpt_StDev)

Both should be >= 1.33; and bias (XBar-RefValue) not significant.

B. Based on GRR, typically two indexes are recommended:

1. Measurement Capability Index (MCI_I) based on Process Variation is defined as:
MCI_I = 100 (Sigma_GRR/Sigma_process)
Sigma_Process is calculated from ongoing SPC studies: RBar/d2

2. The other index, (MCI_II) is based on tolerance:
MCI_II = 100 (5.15 * Sigma_GRR/ Tolerance )
In place of 5.15 you can also use 6 to cover 99.73% spread.

Acceptance criteria for both these are:
<=20% :Good
>20 - <=30% :Marginal
>30% :Unacceptable

Atul,

I am familiar with performing a graphical MSA and developing control charts for Range and X-bar, as well as a Bias chart. I calculate the difference between the UCL and LCL on the X-bar chart to determine a GR&R as % of tolerance similar to your MCI_II above:

GR&R as a % of Tolerance = 100X (UCL - LCL)/Tolerance

In your opinion, is this method similar to your MCI_II method in terms of results?

I use the following Acceptance Criteria:
<=10% :Good
>10 - <=30% :Marginal
>30% :Unacceptable

Thank you very much for your input.

.... I calculate the difference between the UCL and LCL on the X-bar chart to determine a GR&R as % of tolerance similar to your MCI_II above:

GR&R as a % of Tolerance = 100X (UCL - LCL)/Tolerance

In your opinion, is this method similar to your MCI_II method in terms of results?

I use the following Acceptance Criteria:
<=10% :Good
>10 - <=30% :Marginal
>30% :Unacceptable


Not quite, IMO. The XBar chart limits are based on repeatability. Also the (UCL-LCL) will give you 6*Sigma-XBar which will be scaled down from Repeatability stDev by a factor of sqrt(n), where n=subgroup size.

You might want to try out with a few data sets to see what kind of results this equation yields- esp if there is a high reproducibility error.

The 10:1 rule and GR&R results are not directly related. The former says that the device used should be 10x as sensitive as the measurement required. In other words, if you have need to measure to .001", the device should be capable of .0001" discrimination. The 10% GR&R target takes other factors into consideration, such as operator error and part-to-part variation. Whether or not 10% is appropriate is dependent on the criticality of the measurements and the amount of tolerance available. If the process is already using 95% of the tolerance, 10% might be too much. On the other hand, if the process is using only 10% of the tolerance, more gage error is tolerable.

Jim,

When calculating the process variation using a given set of measuring devices, the figure arived at already includes both process variation as well as measurement variation. The acceptability or otherwise of the process is defined using this combined figure.

MSA is a separate analysis where process variation is excluded from the calculation and only measuring system variation is considered. The acceptability or otherwise of this result is an absolute decision. Not related to the process capability.

The example given by you is extreme. A 95% process variation by itself is not acceptable.

Going to other extreme, if the process variation is only 10%, that cannot make a GRR of 50% acceptable.

When calculating the process variation using a given set of measuring devices, the figure arived at already includes both process variation as well as measurement variation.

I think you might need to back up a little and reconsider what you wrote. Did you really mean to say that calculation of process variation assumes knowledge ("...already includes...") of process variation? How can you know something before you know it?

MSA is a separate analysis where process variation is excluded from the calculation and only measuring system variation is considered.

Sorry, potdar, but process variation may or may not be included in calculations, depending on what one hopes to accomplish by the analysis.

The example given by you is extreme. A 95% process variation by itself is not acceptable.

I referred to a process that is using 95% of the tolerance, which is a common occurrence, not "95% process variation."

Going to other extreme, if the process variation is only 10%, that cannot make a GRR of 50% acceptable.

My point was that measurement system evaluation should take into account the amount of the tolerance that's being used by the process. Of course, the AIAG methods leave us in a sort of Catch-22 in this regard, as we can't be sure of the process variation if we measure using systems that have not been qualified.

In most industries (except for AIAG documents) the goal of a measurement system is to be adequate for its intended use. Therefore, the tolerance determines the capability of a device. A ruler is sufficient to measure the size of a nail, but insufficient to measure the distance from Maine to California. Additionally, I concentrate with my clients on sources of variation and not total variation. Knowing the GRR% or other "total system" error does not give any information if the measurement system can be improved to be used in a specific application.

In most industries (except for AIAG documents) the goal of a measurement system is to be adequate for its intended use. Therefore, the tolerance determines the capability of a device.

I think the AIAG methods are aimed at measurement efficacy, but the AIAG processes as a whole (APQP, SPC, MSA) were not well thought out, and sometimes it's obvious that the people who wrote one manual weren't talking to the ones that wrote another.

A ruler is sufficient to measure the size of a nail, but insufficient to measure the distance from Maine to California.

It works OK for me; Los Angeles is 9" away from Bangor on my map.:cool:

I think the AIAG methods are aimed at measurement efficacy, but the AIAG processes as a whole (APQP, SPC, MSA) were not well thought out, and sometimes it's obvious that the people who wrote one manual weren't talking to the ones that wrote another.



It works OK for me; Los Angeles is 9" away from Bangor on my map.:cool:

:)
Jim, you and I think alike....

I was hoping someone would have said you can use a ruler and a map to determine the distance. Of course the discrimination of that measurement system is questionable, but nonetheless you can use it.

I think you might need to back up a little and reconsider what you wrote. Did you really mean to say that calculation of process variation assumes knowledge ("...already includes...") of process variation? How can you know something before you know it?

Well Jim, when I start I only know that the process varies and my measuring system varies. I dont have the faintest idea about "how much".

There also is some confusion regarding the definition of "process variation". Is it "variation due to the process", or "variation in process parameter measurements obtained during analysis"?

The second case is commonly understood as "process variation" and includes variation due to all aspects involved in the process and inspection.

Sorry, potdar, but process variation may or may not be included in calculations, depending on what one hopes to accomplish by the analysis.

The measurement data includes both "variation due to process" and "variation due to MS".

So far as I understand, MSA is necessarily done by eliminating effects of process variation. The calculation methods enable that. There is no relation to "what one hopes to accomplish".

If the data is analysed without any filtration technique, the output is known as the "process variation" (typically variance, Cp,...) by the world. This may be internally analysed further by splitting into various components. Any outside party is hardly ever interested in the breakup.

I referred to a process that is using 95% of the tolerance, which is a common occurrence, not "95% process variation."

Sorry, I may have used wrong words, but 95% of tolerance being used by process variation is a common occurance. And commonly, it is considered unacceptable.

My point was that measurement system evaluation should take into account the amount of the tolerance that's being used by the process. Of course, the AIAG methods leave us in a sort of Catch-22 in this regard, as we can't be sure of the process variation if we measure using systems that have not been qualified.


Exactly what I tried to communicate.

Only, MS variation, being a part of overall process variation should remain limited to an acceptable portion of overall process variation. Thats why if process variation covers 10% of tolerance, MS variation cannot be allowed to cover 50%.

I would like to contribute a few commonplace "measuring systems" as steven has introduced.

- a surface plate and a height gauge

- a vee block, a master and a dial

in such cases we do a detailed study before reaching a conclusion on the acceptabilty of the system. If not acceptable, we go deeper componentwise to find out what exactly is causing how much trouble.

Dear Atul,

Thanks for the clarifications.

We were recently audited by Robert Bosch, and the auditor told us to measure Cg & CgK first for finding the capability of the measuring instrument and then go ahead with doing the measurement of the parts.

I am yet to get an answer for the same.

But since I am not a statistics student I am not able to understand your points. Could you please explain by way of example using an excel sheet?


Sriram

I don't think we're disagreeing here, potdar, just having a little trouble with the language.:D Perhaps my only quibble is this:


So far as I understand, MSA is necessarily done by eliminating effects of process variation. The calculation methods enable that. There is no relation to "what one hopes to accomplish".


And again, perhaps I'm misunderstanding you. The results of GR&R may be used to help determine how a particular gage is to be used (i.e., in accordance with what one hopes to accomplish). Quoting the AIAG MSA manual, Third Edition, page 13:
For product control, variability of the measurement system must be small compared to the specification limits. Assess the system to the feature tolerance.

For process control, the variability of the measurement system ought to demonstrate effective resolution and be small compared to manufacturing process variation. Assess them measurement system to the 6-sigma process variation and/or Total Variation from the MSA study.(Emphasis in the original)

I don't think we're disagreeing here, potdar, just having a little trouble with the language.:D

Yes Jim,

The issues are settled. A gauge that may not be fit for process control may be perfectly OK for incoming inspection of the same spec.:agree1:

I think I shall take some efforts on polishing my presentation.:)

Dear Atul,

Thanks for the clarifications.

We were recently audited by Robert Bosch, and the auditor told us to measure Cg & CgK first for finding the capability of the measuring instrument and then go ahead with doing the measurement of the parts.

I am yet to get an answer for the same.

But since I am not a statistics student I am not able to understand your points. Could you please explain by way of example using an excel sheet?


SriramHi Sriram,
Sorry, I do not have any Excel sheets for this. However, I do know that Bosch has defined procedures, flowcharts and also worksheets for calculating measurement system capability. You can get these documents directly from them. The formulas I gave above were from my very old notes from my interaction with them at MICO.

There also is some confusion regarding the definition of "process variation". Is it "variation due to the process", or "variation in process parameter measurements obtained during analysis"?

The second case is commonly understood as "process variation" and includes variation due to all aspects involved in the process and inspection.



I am still thinking on what's going on here,,,Potdar/Jim if you can explain this difference in terminology used above for the GRR...I know in the GRR calculation Process variation (TV)= sqrt((part var.)^2 +(RR)^2)) .... So I think as Potdar said earlier, the process var. is included in the %GRR calculation (%GRR=RR/TV)...So why don't the rules <10%,etc. apply here?:confused: Any examples to cite here?

This may sound dumb, but any suggestions for this question??

I am still thinking on what's going on here,,,Potdar/Jim if you can explain this difference in terminology used above for the GRR...I know in the GRR calculation Process variation (TV)= sqrt((part var.)^2 +(RR)^2)) .... So I think as Potdar said earlier, the process var. is included in the %GRR calculation (%GRR=RR/TV)...So why don't the rules <10%,etc. apply here?:confused: Any examples to cite here?

This may sound dumb, but any suggestions for this question??

If you are talking about the age old thumb rule of 10:1 resolution, it speaks of least count.

GRR talks of measurement error. As you have rightly put, TV consists of PV and RR. SPC techniques are based on tracking the TV and containing it for process control. For the TV to sufficiently accurately reflect PV, the RR component should be limited to a small fraction of TV. Thats why GRR.

Hope I got you right and you get me right.:D

Yes, I got you but a a question from your reply:

What is the difference between 10:1 rule, P/T ratio and %GRR calculated over total tolerance???

The AIAG manual specifies some criteria for %GRR (over tolerance) like <10%, 10-30%, & >30% for gage selection which says about the discrimination of the gage, where <10% is considered ok.

10:1 rule says a similar thing about the discrimination of the gage to be atleast 1/10 of the total tolerance.

P/T (Precision/Tolerance) also talks about the same.

To me all means the same.

Hope I am not missing anything important here.

Thanks in advance.

Hello everybody,

my question concerns p191 of MSA 3rd edition where the link between Cp and %GRR is given.

When analysing the process capability of a supplier, is it acceptable to back calculate the Cpact of the process from the Cpobs and %GRR determined during the MSA and then present the results as the "actual " process of the supplier to our customers?

If yes, shall we use the %GRR to process or the %GRR to tolerance determined on Incomming Inspection measurment data?

Thans a lot for your help

regards

my question concerns p191 of MSA 3rd edition where the link between Cp and %GRR is given.

When analysing the process capability of a supplier, is it acceptable to back calculate the Cpact of the process from the Cpobs and %GRR determined during the MSA and then present the results as the "actual " process of the supplier to our customers?

If yes, shall we use the %GRR to process or the %GRR to tolerance determined on Incomming Inspection measurment data?
You could definitely provide this information supplementally to the customer, but I do not recommend portraying it as what the customer would normally receive from the typical supplier. Be specific about what was done and how.

In the calculation, you do not use either %GRR. You use the standard deviation. StDevActual = SQRT[(StDevObs)^2 - (StDevMeas)^2]. Then use the StDevActual in your calcualtion of Cp/Cpk.