Thank you very much for your response. The balance that is used is digital. It is considered to be the "Standard" balance for weight calibrations. For a given weight calibration, the same balance is always used, i.e., for all 1 gram weights, a particular Mettler or Sartorius balance is used.
Calculations at this point are based on three repeats of six comparisons of working standard to lab standard. Here's an example of the data
A&B DIFF 0.00024
MEAN A 0.99994
MEAN B 1.00018
Uncertainty was calculated by determining the standard deviation of the lab standard's weight (not the uncertainty of the differences between the lab standard and the reference) and combining that with the standard deviation of the reference.
I see referenced in EA-4/02 that the difference within each ABBA grouping should be considered a single observation. If I directed the lab in that direction, there would only be three observations. Since they calibrate many similar sets of weights to the same reference, can I pool the standard deviations from similar comparisons (e.g., all similar 1 gram weight references)?
Sorry this is so long, I really appreciate comments and responses that I have gotten.