= Measurement uncertainty
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<Measurement uncertainty> describes the spread of values reasonably attributable to a specified <measurand>, using the available information. It differs from <measurement error>, which is a difference from a reference value: the uncertainty can be quantified even when that difference is unknown. A useful result therefore states what was measured, the <unit of measurement>, the value, and how its uncertainty was evaluated.<Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results by Barry N. Taylor and Chris E. Kuyatt>[sections 2 and 7; Appendix D.1]{ref}
For a wire's <electrical resistance> obtained from $R=V/I$, uncertainty in the <voltage> and <electric current> propagates through that <measurement model>. Shared <measurement calibration> effects can make the inputs correlated, so adding percentages or treating every contribution as independent can be wrong. The <measurement uncertainty budget> records those assumptions. Report a <standard uncertainty> when a standard-deviation scale is intended, or an <expanded uncertainty> with its stated <coverage factor>; a bare plus-or-minus number does not specify which was used.
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