Systematic Error 

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.

Taking the mean from a set of repeated readings will reduce the random error but not the systematic error. Random errors are as likely to be above the true value as below it and so will tend to cancel each other out. Systematic errors are always one side of the true value and so will not tend to cancel each other out.