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Biases are caused by inadequacies in the design, conduct, analysis, reporting, or interpretation of treatment comparisons.
Because it is generally not possible to know the degree to which an effect estimate is biased, judgements must be made about the risk of bias using criteria that assess factors that are known or thought to be associated with bias, such as unconcealed allocation of participants to treatments (see allocation schedule concealment).
A high risk of bias is one reason for there being less certainty of the evidence for an effect estimate.
In everyday speech, bias has other meanings, for example ‘ prejudice’.
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