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Uncertainty components are combined in quadrature as standard uncertainties. A resolution of d contributes a rectangular distribution of half-width d/2, so its standard uncertainty is d/(2 x sqrt 3). Multiplying the combined standard uncertainty by a coverage factor of 2 gives roughly 95% confidence.
Combined uncertainty
uc = sqrt(u1^2 + u2^2 + u3^2)
Expanded uncertainty
U = k x uc, with k = 2 for approximately 95% coverage
A reading quantised to steps of d could be anywhere within d/2 of the true value, uniformly distributed. The standard deviation of a rectangular distribution of half-width a is a/sqrt 3.
Yes for conformity assessment. A guard band equal to the expanded uncertainty is applied inside the specification limits so that accepted parts are conforming with the stated confidence.