Measurement BIAS Study - No Measurement Variation Unacceptable?

C

CarlDaniel

#1
I just did a bias study on one of our chip scale package automated vision system. I used an excel template with formula based from MSA manual for bias independent sampling method. I took 15 readings and all were exactly the same with no rounding off. But when I input the readings, it turned out the lower & upper values of the 95% confidence interval of the bias are the same value which would result in a failed bias study.
Please help me understand what happened because based on the condition given for bias study, the bias should be unacceptable eventhough the average of the readings are close to the reference value. I checked the formula and I didn't see any discrepancy, or maybe I missed something.
 

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Jerry Eldred

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#2
Re: BIAS study - No measurement variation still unacceptable?

I'm sure one of the MSA / GR&R experts will give the correct answer, but my less-than-expert guess would be your resolution. I seem to recall from my old QS9000 days, a P/T ratio requirement (precision / tolerance ratio) of at least 10:1. Perhaps you need enough resolution to actually show variation.
 

Miner

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#3
Re: BIAS study - No measurement variation still unacceptable?

No, resolution is not the issue here. The gage has a resolution of .001 compared to a tolerance range of .16.

The formula for the Bias study compares the measured bias against the measurement variation. If the measured bias falls within the distribution of measurement error, the gage cannot really see the bias, so it is considered acceptable.

If the measured bias falls outside the distribution of measurement error, the gage can see the bias, so it is considered unacceptable.

In this case, the gage displayed zero measurement variation, so it saw the bias of -.003. If you went to a gage with a resolution of .0001, you may see measurement variation, but the spread would undoubtedly be so tight that the .003 bias would still be outside the measurement error.

Your decision on how to react should consider three things:
1) Is a bias of -.003 outside your acceptance limits for calibration?

If your answer to 1) is NO, do nothing. If the answer is YES, recalibrate the gage and perform a risk assessment as follows:

2) Will a bias of -.003 cause you to ship nonconforming product?
3) Will a bias of -.003 cause you problems with process control?

If your answer to 2) and/or 3) is NO, document the decision process and move forward. If it is YES, follow your procedures for shipment of suspect product.
 
C

CarlDaniel

#4
Re: BIAS study - No measurement variation still unacceptable?

Thanks a lot Miner...the manual just didn't explain this thought process you presented.

The -0.003 variation is a negligible amount and won't affect our product.
 
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