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Definitions
The standard uncertainty u(y) of a measurement result y is the estimated standard deviation of y.

The relative standard uncertainty ur(y) of a measurement result y is defined by ur(y) = u(y)/|y|, where y is not equal to 0.

Meaning of uncertainty
If the probability distribution characterized by the measurement result y and its standard uncertainty u(y) is approximately normal (Gaussian), and u(y) is a reliable estimate of the standard deviation of y, then the interval y u(y) to y + u(y) is expected to encompass approximately 68 % of the distribution of values that could reasonably be attributed to the value of the quantity Y of which y is an estimate. This implies that it is believed with an approximate level of confidence of 68 % that Y is greater than or equal to y - u(y), and is less than or equal to y + u(y), which is commonly written as Y= y ± u(y).

Use of concise notation
If, for example, y = 1 234.567 89 U and u(y) = 0.000 11 U, where U is the unit of y, then Y = (1 234.567 89 ± 0.000 11) U.  A more concise form of this expression, and one that is in common use, is Y = 1 234.567 89(11) U, where it understood that the number in parentheses is the numerical value of the standard uncertainty referred to the corresponding last digits of the quoted result.