[Risk of Bias Across Study Designs]

Bias means a systematic error and is a problem in research. Several bias tools are available, which are specific and depend on the study design. There are four overall categories of bias: selection bias, information bias, reporting bias, and other bias. This review summarises bias assessment in randomised controlled trials, cohort studies, and systematic reviews. Assessment of bias can be used at any point in the research process, not only for published research articles. This is an important step to avoid research waste.