Error
Error is the unobserved difference between a true value and its measurement or observation.
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Description
Given a set of observations, the expected value can be estimated. The difference between each observation and that expected value is an error. Importantly though, the true mean of the population is generally unobservable, so the true error is also unobservable.
Compare to residuals, which are the observed difference between an observation and an estimate or prediction for the true value.