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In metrology, measurement uncertainty is a non-negative parameter characterizing the dispersion of the values attributed to a measured quantity. The uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity. All measurements are subject to uncertainty and a measured value is only complete if it is accompanied by a statement of the associated uncertainty. Relative uncertainty is the measurement uncertainty divided by the measured value. ==Background== The purpose of measurement is to provide information about a quantity of interest – a measurand. For example, the measurand might be the size of a cylindrical feature per ASME Y14.5-2009, the volume of a vessel, the potential difference between the terminals of a battery, or the mass concentration of lead in a flask of water. No measurement is exact. When a quantity is measured, the outcome depends on the measuring system, the measurement procedure, the skill of the operator, the environment, and other effects.〔(Bell, S. Measurement Good Practice Guide No. 11. A Beginner's Guide to Uncertainty of Measurement. Tech. rep., National Physical Laboratory, 1999. )〕 Even if the quantity were to be measured several times, in the same way and in the same circumstances, a different measured value would in general be obtained each time, assuming the measuring system has sufficient resolution to distinguish between the values. The dispersion of the measured values would relate to how well the measurement is performed. Their average would provide an estimate of the true value of the quantity that generally would be more reliable than an individual measured value. The dispersion and the number of measured values would provide information relating to the average value as an estimate of the true value. However, this information would not generally be adequate. The measuring system may provide measured values that are not dispersed about the true value, but about some value offset from it. Take a domestic bathroom scale. Suppose it is not set to show zero when there is nobody on the scale, but to show some value offset from zero. Then, no matter how many times the person's mass were re-measured, the effect of this offset would be inherently present in the average of the values. Measurement uncertainty has important economic consequences for calibration and measurement activities. In calibration reports, the magnitude of the uncertainty is often taken as an indication of the quality of the laboratory, and smaller uncertainty values generally are of higher value and of higher cost. The American Society of Mechanical Engineers (ASME) has produced a suite of standards addressing various aspects of measurement uncertainty. ASME B89.7.3.1, Guidelines for Decision Rules in Determining Conformance to Specifications addresses the role of measurement uncertainty when accepting or rejecting products based on a measurement result and a product specification. ASME B89.7.3.2, Guidelines for the Evaluation of Dimensional Measurement Uncertainty, provides a simplified approach (relative to the GUM) to the evaluation of dimensional measurement uncertainty. ASME B89.7.3.3, Guidelines for Assessing the Reliability of Dimensional Measurement Uncertainty Statements, examines how to resolve disagreements over the magnitude of the measurement uncertainty statement. ASME B89.7.4, Measurement Uncertainty and Conformance Testing: Risk Analysis, provides guidance on the risks involved in any product acceptance/rejection decision. The "Guide to the Expression of Uncertainty in Measurement", commonly known as the GUM, is the definitive document on this subject. The GUM has been adopted by all major National Measurement Institutes (NMIs), by international laboratory accreditation standards such as ISO 17025 which is required for international laboratory Accreditation, and employed in most modern national and international documentary standards on measurement methods and technology. See Joint Committee for Guides in Metrology. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「measurement uncertainty」の詳細全文を読む スポンサード リンク
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