measurement 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, for example, 32.3 ± 0.5 cm.

Scientific observations are marred by two distinct types of errors, systematic errors on the one hand, and random on the other hand. The effects of random errors can be mitigated by repeated measurements. Constant or systematic errors on the contrary must be carefully avoided, because they arise from one or more causes which constantly act in the same way, and have the effect of always altering the result of the experiment in the same direction. They therefore alter the value observed and repeated identical measurements do not reduce such errors.