Uncertainty in measurements.

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Measurement Uncertainty
An Approach via the Mathematical Theory of Evidence
Simona Salicone
© 2007 Springer Science+Business Media, LLC

1 Uncertainty in Measurement
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1.2 The Theory of Error
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…errors are traditionally divided into two general classes: random and systematic.
Random errors occur with different values and signs in each different measurement (of the same quantity, with the same reference conditions). However, if the measurement of the same quantity is repeated a sufficient number of times N, the mean of these errors tends to zero and the mean of the measurement results tends to the true value of the quantity. Random errors presumably arise from unpredictable or stochastic temporal and spatial variations of the influence quantities.
The random error of a measurement result cannot be compensated by applying a correction, but it can be usefully reduced by increasing the number of observations.
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…systematic errors have the following characteristic: When the measurement of a certain quantity is repeated, following the same measurement process, with the same measurement instruments and the same reference conditions, they always occur with the same value and sign.
From a strict theoretical point of view, systematic errors, differently from random ones, can be fully compensated, and their effect can be eliminated. This result may happen, however, only if the systematic errors are totally recognized.
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