A bias is a systematic error in an aspect of a study, operating in either direction. “Systematic error” means that even if the biased study is replicated many times, the wrong result will still be reached on average. This is different to imprecision, which refers to random errors in the conduct of the study, meaning that multiple replications of the study would deliver results that form a distribution centring on the true population value. A bias can be so small as to have no impact on the observed effect, or so large that what appears to be an effect is in fact entirely due to bias. A rigorous systematic review should assess the risk of bias of all studies in the review, as this will have an impact upon the reliability of the review’s conclusions. Bias can appear at any point in a study or trial, for example in the randomisation of participants (“selection bias”), and the choice of which findings to report (“reporting bias”). A more comprehensive overview of the range of possible biases is provided by the Cochrane Collaboration (read more about the Risk of Bias Tool here).

How to cite: Bias [online]. (2016). York; York Health Economics Consortium; 2016. https://www.yhec.co.uk/glossary/bias/

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