Tab. I.

Types of bias in clinical research.

Type of Bias	Description
Investigator bias	Conscious or unconscious preference given to one group over another by the investigator
Evaluator bias	Introduced when an investigator making endpoint-variable measurements favours one group over another. Common with subjective endpoints
Performance bias/ set of Hawthorne effects	Introduced when participants know their allocation to a particular group and change their response or behaviour during a particular treatment
Selection bias	Introduced when samples (individuals or groups) are selected for data analysis without proper randomization; includes admission bias and non-response bias, in which case the sample is not representative of the population
Ascertainment or information bias	Errors in measurement or classification of patients; includes diagnostic bias and recall bias
Allocation bias	Systematic differences in the allocation of participants to treatment groups and comparison groups, when the investigator knows which treatment is going to be allocated to the next eligible participant
Confirmation bias	Information is processed in a manner consistent with someone’s belief
Belief bias	The strength of arguments is judged on the basis of the plausibility of their conclusions rather than how strongly they support that conclusion.
Expectation bias	Introduced during publication by a personal preference for positive results over negative results when the results deviate from expected outcomes
Detection bias	Systematic errors in observation of outcomes in different groups results in detection bias when outcomes in one group are not as vigilantly sought as in the other.
Attrition bias/loss-to-follow-up bias	Preferential loss-to-follow-up in a particular group leads to attrition bias.
Commercial bias	Introduced for commercial reasons in the form of advertising or economic pressure on editors, particularly in studies involving new medical devices and drugs