Bayes’ theorem: A theorem used to update the probability of an event in the light of a piece of new evidence.6

Bayesian analysis: A statistical approach based on Bayes’ theorem that can be used in single studies or meta-analysis.6 Bayesian analysis involves the use of existing and new information to estimate the risk that a person will experience an event.1

Beta error: See Type II error.

Bias: A systematic error in study design or conduct that results in a distorted assessment of the intervention’s impact on the measured outcomes.1 In clinical trials, the main types of bias arise from systematic differences in study groups that are compared (selection bias), exposure to factors apart from the intervention of interest (performance bias), participant withdrawal or exclusion (attrition bias), or assessment of outcomes (detection bias).6 Reviews of studies may also be particularly affected by reporting bias, where a biased subset of all the relevant data is available.6

Bias prevention: Aspects of study design or conduct intended to prevent bias. In clinical trials, such aspects include randomization, blinding, and concealment of allocation.6

Blinded study: An experimental study in which participants do not know the treatment they are receiving; investigators may also be blind to the specific treatments.4 Double blind means that neither participants nor investigators know which treatment the participants receive.4 However, the terms single blind, double blind and triple blind are not used consistently and are ambiguous unless those who are blinded are specified.6

Blinding: See blinded study.