Means that across many observations there is a deterministic pattern of errors whose amplitude is stochastically drawn from an underlying probability distribution; “structured random” therefore implies “unpredictable” and “correlated across measurements”; the degree of “averaging out” across many measured values depends on the structure of the effect across those measured values; structured random effects may be operating at the same time as other types of effect, in which case only a component of the total error is structured random; an example of a structured random effect is the impact of a random error in the measurement of signal while viewing a calibration target, which causes unpredictable but inter-related errors in all measured values which use that calibration cycle.
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