NIST SOP 29 - Assignment of Uncertainty - Question

greif

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After much searching, I found this 2018 NIST document (SOP 29) which details step by step understandable directions for figuring uncertainty.

It can be found here:
https://www.nist.gov/sites/default/...sop-29-assignment-of-uncertainty-20180131.pdf

My Question:

In section 3.2.5.2.4, (significance level) it says that if bias (machine error) is larger than the combined uncertainty it must be investigated.
I don't understand the "Criteria Limit"; what is x-bar(lab)?
what is x-ref?

Thanks!
 
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