What does the uncertainty mean? Why do some values come with no uncertainty?
A. The uncertainty reflects the errors associated with the certification of the composition for a particular element. Statistically, it contains the errors due to inhomogenity, instrumental precision and bias between analytical techniques used in the certification processes. In most cases the predominate error is the between technique bias. The given limits of uncertainty represent this combined uncertainty and seek to estimate, with a 95% confidence level, a range in which the true value may be expected to lie. In some cases no uncertainty is given because of limited data or the lack of a second independent measure.
