Measurement System Analysis (MSA) - Bias, Linearity and Stability

G

Gator68

#11
Sorry, I don't have an excel template. These are hard numbers to nail down, as people will measure them different ways. For Linearity you typically do a linear fit on the data, then calculate the max deviation from the fit. Finally, you divide by the full scale to get a percent full scale. Bias is usually pretty simple. Take data for some time (say 100 - 1000 samples). Calculate the average value, that's the bias. Stability is very tricky. In the sensor world, one of the standard ways to measure stability is using an Allan variance calculation. See attached for a short description.

Edit -- after looking around the forums a bit, it looks like you're probably talking about measurment errors? Look through some of the related topics, I think this has been covered before. :eek:
 

Attachments

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S

sitapaty

#12
Can some one show excel work sheets to derive BIAS,LINEARITY & Stability please.
Thank you in advance
-Sitapaty
 
T

Tseng

#14
Dear All

May I ask a quite low question regarding linearity example in MSA-4?
The example of Chapter III, section B, p.91 stated ta=-12.043, and tb=10.158....can anyone explain how they were calculated/produced?

Thanks a lot!
 
L

Lala77

#15
hi all..

i've been silently reading all the information in the forum, and still can not get any suitable solution for the problem im facing.

My situation is :

1) Currently we have only 1 inspector to measure the critical part from our product that we decided to do the MSA..from what i know, is it possible to use GR&R for my case since it does not involve more than 1 inspector henc the reproducibility cant be obtained right?

2) But, i tried to add 2 more inspectors just for the GR&R, still its failing.Im guessing that this is because the tool used for measuring is measurement tape and the tolerance is +- 1mm, the readings we get is still between the tolerance and is acceptable, so now im not really sure what should i do get a pass for GR&R.

it would be more helpful if someone from here can enlighten me :(.

Discussed already with the consultant, but he seems to be more blank than me.:frust:
 
D

debyang

#16
Bad GRR result could be possible due to the other two inspectors are not so well trained as the only one who is in charge of the measurement. If you post the GRR results for us to review to understand what might be going wrong.

But I would suggest you do stability study with the only one inspector first to see if the MS is stable and the EV(within system variation) is small enough compaired with the spec tolerance.
 

harry

Super Moderator
#17
................ 2) But, i tried to add 2 more inspectors just for the GR&R, still its failing.Im guessing that this is because the tool used for measuring is measurement tape and the tolerance is +- 1mm, the readings we get is still between the tolerance and is acceptable, so now im not really sure what should i do get a pass for GR&R. .............
I think you've got it. The best estimate for a measuring tape graduated in millimeters is half a millimeter. For +/- 1mm, you need a gage capable of reading 0.1mm.
 

Helmut Jilling

Auditor / Consultant
#18
hi all..

i've been silently reading all the information in the forum, and still can not get any suitable solution for the problem im facing.

My situation is :

1) Currently we have only 1 inspector to measure the critical part from our product that we decided to do the MSA..from what i know, is it possible to use GR&R for my case since it does not involve more than 1 inspector henc the reproducibility cant be obtained right?

2) But, i tried to add 2 more inspectors just for the GR&R, still its failing.Im guessing that this is because the tool used for measuring is measurement tape and the tolerance is +- 1mm, the readings we get is still between the tolerance and is acceptable, so now im not really sure what should i do get a pass for GR&R.

it would be more helpful if someone from here can enlighten me :(.

Discussed already with the consultant, but he seems to be more blank than me.:frust:
Why are you using a measuring tape for a close tolerance of +/- 1mm? The variance on the tip alone can exceed that. A more stable steel rule or square might be more effective?
 

Miner

Forum Moderator
Staff member
Admin
#19
I agree with Harry. Also, you mentioned that only one operator performed this measurement. Is this always the case? In special situations where there is only one operator for a measurement such as with a specialized lab test, you can perform a Gage R study instead of the typical Gage R&R study. The Gage R study evaluates the Repeatability of a single operator.
 
L

Lala77

#20
Bad GRR result could be possible due to the other two inspectors are not so well trained as the only one who is in charge of the measurement. If you post the GRR results for us to review to understand what might be going wrong.

But I would suggest you do stability study with the only one inspector first to see if the MS is stable and the EV(within system variation) is small enough compaired with the spec tolerance.
Thanks debyang, here i've attached the result, i hope anyone could suggest something for me to pass the GRR.:confused:
 

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