Include Type B in effective degrees of freedom for expanded uncertainty? This post has sat unanswered for a couple days in the TalkStats forum so I'm hoping somebody here can help me out.
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.
 A: 
From a plain reading of this step, I understand the combined uncertainty (Uc) to contain all Type A and all Type B contributions

You understand it correctly. The GUM treats type A and type B uncertainties on an equal footing.

except that upon reviewing an example on the NIST site I found that they were excluding the Type B components altogether when calculating Veff.

If you look at the NIST example, they are applying the Welch-Satterthwaite formula (G.2b) as given in the GUM. The $u$ that appears in the numerator of the NIST example is really the combined uncertainty of both type A and type B uncertainty components, even though the subscript "c" is omitted. For the type-B uncertainty components, they assume infinite degrees of freedom so that in the denominator, and just in the denominator, the sum is limited to the type-A components.
A: You can read EA 4/02 "Evaluation of the Uncertainty of Measurement In Calibration", they write"For a standard uncertainty  u(q) obtained from a Type A evaluation as discussed  in  sub-section 3.1,  the  degrees  of  freedom  are  given  by i = n-1. It is more problematic to associate degrees of freedom with a standard uncertainty u(xi) obtained from a Type B evaluation. However, 
it is common practice to carry out such evaluations in a manner that 
ensures that any underestimation is avoided. If, for example, lower and 
upper limits  a–  and  a+  are set, they are usually chosen in such a way 
that the probability of the quantity in question lying outside these limits 
is  in  fact  extremely  small.  Under  the  assumption  that  this  practice  is followed,  the  degrees  of  freedom  of  the  standard  uncertainty  u(xi) 
obtained from a Type B may be taken to be vi -> ifinity." 
