Calculating Bias - Is this method acceptable? MSA manual page 85

D

darkafar

#1
MSA manual page 85 states “…select a production part that falls in the mid-range of the production measurements and designate it as the master sample for bias analysis. Measure the part n≥10 times in the tool room, and compute the average of the n readings. Use this average as the “reference value”.

I have no measuring equipment with better sensitivity. Our workplace is in an air-conditioned clean room. I don’t know what a tool room environment is like. I presuppose our clean room environment match that of a tool room. So I asked an operator to measure one sample 10 times, calculated the average, used it as the reference value, then asked the same operator to measure the sample 15 times, and then I followed the formulates given in the manual to decide if the bias is acceptable.

I did this because I believe bias is an important factor that no reason can justifiably exclude it. I used the same operator to decide the reference value because she used the equipment best, and she usually performed the measuring.

I question is, since I used the same operator, sample, environment, method, equipment, there is no reason why the averages would be different. So the bias analysis was reduced to a ritual.

Do you think my method acceptable? If you are in my shoe, what will you do?
 
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Miner

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#2
No. This method is not acceptable. As you say, it is an excercise in futility, with no value.

If you were evaluating the bias of a shop-floor gage, you could use your clean-room environment as the reference since it sounds better than a tool room. However, you are trying to evaluate the bias of the clean-room. Therefore, your reference needs to be from a higher standard than the clean-room.

In this situation, you really cannot use parts, but should use master standards that have been calibrated at a higher level than your lab is capable of meeting.
 
D

darkafar

#3
But I did it in complete compliance with the MSA manual, didn't I?
The MSA manual gave an example following the statement, and my way of performing the bias analysis is an emulation of the example. The example did not mention a master standard.
If a master standard and a better equipment are both unavailable, do you think it's meaningful to do a bias analysis with my method? How to convince the customer that the bias analysi is unnecessary?
 

Miner

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#4
The main difference between what you did and the example in the MSA manual is that the MSA manual uses a tool room to obtain a theoretically more exact measure of the part's true dimension than that of the gage to be evaluated.

In your case, you are not doing that. You are using the same gage to assess itself. Your bias should be so close to zero as to not matter.

Do you have a better gage or an outside source available?
 
D

darkafar

#5
No. That's my problem. I guess what you mean is that the process to establish the "reference value" have to be different to the normal measuring process, so that the difference can be differentiated.
It really doesn't matter whether it's a better tool room or a better operator, as long as there is difference.
So that when the difference can not be established, it would be meaningless to do the bias analysis unless we want to cheat an overdemanding customer.
Am I right?
 

Miner

Forum Moderator
Staff member
Admin
#6
Exactly correct.

That was why I sugested using a calibrated standard of a high enough class. While it is not the product, the "true" value has been established by a qualified lab.
 
M

martin elliott

#7
I agree with Miner, you cannot assess the bias of a measurement system against itself as a reference.

If its any value at all, my practice has been, for example, when determining the bias on a digital caliper (resolution 0.01 Accuracy 0.02), was to measure the standard part(s) in a more controlled enviroment using digital Micrometers (resolution 0.001 Accuracy 0.003).

If you cannot achieve this youself, you are left perhaps with an exercise of using an independant approved calibration laboratory that can measure the part to the required additional resolution/accuracy to make sense of the bias evaluation.

Having said all that, and not knowing your situation, are you convinced bias is an issue here beyond that reasonably expected to be covered by normal regular calibration of the instrument. If your problem is justifing such calibration periods then stability might be a better measure to consider
 
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