Editing Observational error (section)

====Imperfect calibration====
Sources of systematic error may be imperfect calibration of measurement instruments (zero error), changes in the [[Biophysical environment|environment]] which interfere with the measurement process and sometimes imperfect methods of [[observation]] can be either zero error or percentage error. If you consider an experimenter taking a reading of the time period of a pendulum swinging past a [[fiducial marker]]:  If their stop-watch or timer starts with 1 second on the clock then all of their results will be off by 1 second (zero error). If the experimenter repeats this experiment twenty times (starting at 1 second each time), then there will be a [[percentage error]] in the calculated average of their results; the final result will be slightly larger than the true period.

[[Distance]] measured by [[radar]] will be systematically overestimated if the slight slowing down of the waves in air is not accounted for. Incorrect zeroing of an instrument is an example of systematic error in instrumentation.

Systematic errors may also be present in the result of an [[Computational mechanics|estimate]] based upon a [[mathematical model]] or [[physical law]]. For instance, the estimated [[oscillation frequency]] of a [[pendulum]] will be systematically in error if slight movement of the support is not accounted for.