Observational error

Observational error (or measurement error) is the difference between a measured value of a quantity and its unknown true value. 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. 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. Systematic error may also refer to an error with a non-zero mean, the effect of which is not reduced when observations are averaged. 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.