Observer bias (also called ascertainment or detection bias) is caused when the actions of an investigator affect the results of a trial. Observer bias can be unintentional and could arise from the investigator's hopes or expectations of the trial. It is most likely to occur when:
the investigator's personal judgement is used to measure subjective trial outcomes, and
the investigator knows which group each person is allocated to (for example treatment or placebo).
The most important design technique for avoiding observer bias in clinical trials is blinding. The potential effect of bias should also be taken into account during statistical analysis of trial data.