Experimenter's bias

An example of systematic bias would be a thermometer that always read three degrees colder than the actual temperature because of incorrect initial calibration or labelling, whereas one that gave random values within five degrees either side of the actual temperature would have random error. Once detected, systematic effects are easier to take into account than random effects: in the example just given, if you know that your thermometer always reads three degrees below the correct value, you can simply make a systematic correction by adding three degrees to all readings ...

See also:

Systematic bias, Systematic bias - Systematic vs. random

Read more here: » Systematic bias: Encyclopedia II - Systematic bias - Systematic vs. random