These cause readings to differ from the true value by a consistent amount each time a measurement is made.
Sources of systematic error can include the environment, methods of observation or instruments used.
Systematic errors cannot be dealt with by simple repeats. If a systematic error is suspected, the data collection should be repeated using a different technique or a different set of equipment, and the results compared.
e.g. A systematic error occurs when using a wrongly calibrated instrument.
e.g. Rachael's pendulum timing experiment was made worse by the fact that she also began counting at '1' not '0'. So all her times, in addition to random errors in her counting, were also short of one full swing each making her calculated times all smaller than the 'true values'.