LYCOS RETRIEVER 
Measurement: Uncertainties
built 644 days ago
[T]he reduction—not necessarily the elimination—of uncertainty is central to the concept of measurement. Measurement errors are often assumed to be normally distributed about the true value of the measured quantity. Under this assumption, every measurement has three components: the estimate, the error bound, and the probability that the actual magnitude lies within the error bound of the estimate. For example, a measurement of the length of a plank might result in a measurement of 2.53 meters plus or minus 0.01 meter, with a probability of 99%.
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Uncertainty is the doubt (potential measurement error) associated with the validity of a measurement. In terms of a coating thickness gage, uncertainty identifies the measurement errors that could reasonably occur when measuring the thickness of a coating. This may include the gage uncertainty (repeatability based on gage accuracy at the thickness in question), operator uncertainty (reproducibility based on ability of the operator to influence the readings), temperature and humidity uncertainties (impact of environmental conditions), as well as other application specific uncertainties. A common means for combining these uncertainties is the sum of squares method shown in the following formula.
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Traceability refers to the ability to trace the result of a measurement or calibration to National standards through an unbroken chain of comparisons, each with stated uncertainties. It's another ISO requirement that is often left to an outsourced supplier. Sometimes, though, this can't be done.
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Uncertainties