(See VIM, 3.10, 3.12–3.14; and NCSL pages 11–13.) In metrology, error (or measurement error) is an estimate of the difference between the measured value and the probable true value of the object of the measurement. The error can never be known exactly; it is always an estimate. Error may be systematic and/or random. Systematic error (also known as bias) may be corrected. See also: bias, correction (of error), random error, systematic error.