Bias

Bias refers to a systematic error in a study, skewing results in either direction. ‘Systematic error’ means that the study will consistently reproduce an inaccurate result, regardless of how many times it is repeated. This is different to imprecision, which refers to random errors that average out over multiple replications; the results would form a distribution centering on the true population value. A bias can be negligible and have no impact on the observed effect, or it can be substantial and mask or fabricate a true effect. A rigorous systematic review should assess the risk of bias in the included studies because this will impact the reliability of the review’s conclusions. Bias can occur at any stage of a study or trial, including participant randomisation (selection bias) and selective reporting of findings (reporting bias). For a comprehensive overview of potential biases, refer to the Cochrane Collaboration.