Systematic errors are predictable and typically constant or proportional to the true value. If the cause of the systematic error can be identified, then it usually can be eliminated. An error is called systematic if the functional relationship between the magnitude of the error and the intensity of the physical quantity (that is the cause) is known. Systematic errors always occur with the same sign + or – and the same amplitude, where the measurement of a physical quantity is repeated several times with the same instrumentation and under the same operating and environmental conditions.
Systematic errors are caused by imperfect calibration of measurement instruments or imperfect methods of observation (an error, voluntary or involuntary, committed by the observer), or interference of the environment with the measurement process, and always affect the results of an experiment in a predictable direction.
Incorrect zeroing of an instrument leading to a zero error is an example of systematic error in instrumentation. Other types of errors are: gross errors, static errors, and dynamic errors.