The gap between a recorded value and the true value it's meant to represent.
Measurement error is the difference between an observed score and the true (but unknowable) value, arising from imperfections in the instrument, protocol, rater, or subject. It is typically split into random error (noise that varies unpredictably between trials, reducing reliability) and systematic error or bias (a consistent offset in one direction, e.g. a poorly calibrated device). Because true values can never be directly observed, measurement error is estimated indirectly using methods such as test-retest reliability, standard error of measurement, or limits of agreement.
This guide was auto-drafted and is pending editorial review.