The calculation of uncertainty for a measurement is an effort to set reasonable bounds for the measurement result according to standardized rules. Since every measurement produces only an estimate of the answer, the primary requisite of an uncertainty statement is to inform the reader of how sure the writer is that the answer is in a certain range. This report explains how we have implemented these rules for dimensional calibrations of nine different types of gages: gage blocks, gage wires, ring gages, gage balls, roundness standards, optical flats indexing tables, angle blocks, and sieves.

This paper presents a detailed discussion of the technical aspects of the calibration process with emphasis on the definition of the measurand, the conditions under which the calibration results are valid, and the subsequent use of the calibration results in measurement uncertainty statements. The concepts of measurement uncertainty, error, systematic error, and reproducibility are also addressed as they pertain to the calibration process.Key words: calibration; error; influence quantities; measurand; systematic error; uncertainty. Accepted: January 16, 2001Available online: http://www.nist.gov/jres IntroductionThe concept of calibration has generally been associated with statements regarding the accuracy of a standard, gauge, or measuring instrument. Although calibration typically involves many administrative, procedural, and documentary activities [1][2][3], in this paper we will focus on technical issues associated with measurement error and uncertainty as it relates to the calibration process. Modern metrological concepts increasingly link the topics of measurement traceability, laboratory accreditation, and quality assurance programs to the topic of measurement uncertainty. An essential component of all uncertainty budgets is the employment of calibrated gauges, standards, or instruments. It is the calibration process that transfers a reference value, usually an International System (SI) unit, to the artifact or instrument under calibration and hence establishes the "unbroken chain of comparisons" required for traceability. Calibration (VIM-1993)-set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and the corresponding values realized by standards.1 Even calibrations that use "self-calibration" methods, e.g., straightedge reversal using an indicator, require uncertainty statements since the uncertainty of the indicator must be assessed. 2. A calibration may also determine other metrological properties such as the effect of influence quantities.

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