A2LA Calibration Program Requirements, September 2005 II. Section 2.1.3.b.3

S

Stephen Rubino

#1
A2LA Calibration Program Requirements, September 2005 II. Section 2.1.3.b.3 requires, “when parameters are certified to be within specified tolerance, the associated uncertainty of the measurement result is properly taken into account with respect to the tolerance by a documented procedure or policy established and implemented by the laboratory that defines the decision rules used by the laboratory for declaring in or out of tolerance conditions.”

What are they really looking for here? :)
 
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Wesley Richardson

Wes R
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#2
A2LA Calibration Program Requirements, September 2005 II. Section 2.1.3.b.3 requires, “when parameters are certified to be within specified tolerance, the associated uncertainty of the measurement result is properly taken into account with respect to the tolerance by a documented procedure or policy established and implemented by the laboratory that defines the decision rules used by the laboratory for declaring in or out of tolerance conditions.”

What are they really looking for here? :)
Hi Stephen,

Let me first state that I used to work at an A2LA Accredited laboratory, but I have not been involved with that company for about six years.

They are saying they want a document procedure that defines how your laboratory concludes whether a measurement meets or does not meet the specification. This procedure is a statement of your decision rule, that includes the effect of measurement uncertainty.

For any measurement, there is a measurement uncertainty. Let a specification be 0.995 in to 1.005 in. Assume that I have determined the measurement uncertainty at a 95% confidence, when measuring a nominal 1.000 in dimension, to be 0.002 in, or the true value may be 0.998 in to 1.002 in.

Case 1. My measurement is 1.000 in, then I conclude the true value of the measurement is within specification.

Case 2. My measurement is 1.010 in, then I conclude the true value of the measurement is out of specification.

Case 3. My measurement is 1.006 in, my decision rule states whether I can consider the measurement uncertainty of 0.002 in, and conclude the measurement is within specification, or my decision rule states that the measurement is out of specification.

Case 4. My measurement is 1.004 in, my decision rule states whether I can consider the measurement uncertainty of 0.002 in, and conclude the measurement is out of specification, or my decision rule states that the measurement is within specification.

This is also known as guardbanding. The decision rules are dependent on the impact of a wrong decision, and most likely are different for different types of products. Having a number 2 pencil be a little long or a little short, compared with the specification, has a much different impact, than the length of a jet engine turbine blade.

If your laboratory is an independent testing and/or calibration laboratory, your decision rules may also need to reflect what your specific client requires for the part you test or calibrate for them. This is because your clients may have much different results, depending if a good item is considered bad, or a bad item is considered good.

The simple days of good or bad, black or white, in tolerance or out of tolerance, are gone.

Wes R.
 
S

Stephen Rubino

#3
Thank you Wes!

I suspected as much - it means basically defining the parameters of in/out of tolerence and bringing in a "5 Whys" methodology to qualify how the Technician would determine same and address it for internal application or, as you say, to account for possible impact on external parties (such as when calibrating for a customer).


Thanks again!

Steve
 
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