B

Baseline characteristics
Values of demographic, clinical and other variables collected for each participant at the beginning of a trial, before the intervention is administered.

Bias
[In statistics.] A systematic error or deviation in results or inferences from the truth.  In studies of the effects of health care, the main types of bias arise from systematic differences in the groups that are compared (selection bias), the care that is provided, exposure to other factors apart from the intervention of interest (performance bias), withdrawals or exclusions of people entered into a study (attrition bias) or how outcomes are assessed (detection bias). Reviews of studies may also be particularly affected by reporting bias, where a biased subset of all the relevant data is available.

Bias prevention
Aspects of the design or conduct of a study designed to prevent bias. For controlled trials, such aspects include randomisation, blinding and concealment of allocation.

Blinding (synonym: masking)
[In a controlled trial:] The process of preventing those involved in a trial from knowing to which comparison group a particular participant belongs. The risk of bias is minimised when as few people as possible know who is receiving the experimental intervention and who the control intervention. Participants, caregivers, outcome assessors, and analysts are all candidates for being blinded.  Blinding of certain groups is not always possible, for example surgeons in surgical trials. The terms single blind, double blind and triple blind are in common use, but are not used consistently and so are ambiguous unless the specific people who are blinded are listed.  (Also called masking.)