Expanded uncertainty and Type B

#1
Include Type B in effective degrees of freedom for expanded uncertainty?

I'm looking for assistance in clarifying how Type B uncertainty is incorporated into the determination of expanded uncertainty. I'm using the Guide to Uncertainty in Measurement (GUM) as the primary reference. This lays out the steps for calculating expanded uncertainty in G.6.4.

In particular, I need input on my interpretation of step 2, computation of the effective degrees of freedom (Veff) through the Welch-Satterthwaite formula (G.2b). From a plain reading of this step, I understand the combined uncertainty (Uc) to contain all Type A and all Type B contributions. In the denominator, the degrees of freedom for individual Type B components (Vi) is often set to infinity (as would be the case for a half-width from an equipment data sheet). This takes that term to zero, though the uncertainty component is still part of the combined uncertainty in the numerator.

I'd be comfortable with this interpretation, except that upon reviewing an example on the NIST site I found that they were excluding the Type B components altogether when calculating Veff.

Can anybody identify which technique is correct? Thanks for the help.
 
Last edited: