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Biases (systematic errors) distort effect estimates away from the actual effect.
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).
In everyday language, bias has other meanings, for example ’prejudice’.
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