The GUM distinguishes Type A and Type B methods for evaluating uncertainty. A Type A method uses statistical analysis of repeated observations. Usually this is used to estimate the uncertainty associated with random effects. It is possible to use Type A methods to estimate the uncertainty associated with effects that are systematic for the measurement of interest but consciously randomised for the purposes of uncertainty evaluation (e.g. by realigning an instrument that would normally not be realigned, or varying a temperature that would normally be constant). In Earth Observation Type A methods are generally used to estimate noise statistics – a random effect process.

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