Observational error

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Observational error (or measurement error) is the difference between a measured value of a quantity and its unknown true value.^[1] Such errors are inherent in the measurement process; for example lengths measured with a ruler calibrated in whole centimeters will have a measurement error of several millimeters. The error or uncertainty of a measurement can be estimated, and is specified with the measurement as, for example, 32.3 ± 0.5 cm. (A mistake or blunder in the measurement process will give an incorrect value, rather than one subject to known measurement error.)

Measurement errors can be divided into two components: random and systematic.^[2] Random errors are errors in measurement that lead to measurable values being inconsistent when repeated measurements of a constant attribute or quantity are taken. Systematic errors are errors that are not determined by chance but are introduced by repeatable processes inherent to the system.^[3] Systematic error may also refer to an error with a non-zero mean, the effect of which is not reduced when observations are averaged.^{[citation needed]} For example, length measurements with a ruler accurately calibrated in whole centimeters will be subject to random error; a ruler incorrectly calibrated will also produce systematic error.

Measurement errors can be summarized in terms of accuracy and precision. Measurement error should not be confused with measurement uncertainty.