For example if the most precise studies tended to each have a score of 1 for that outcome, the overall score for that outcome would tend towards 1. Table 3: Principle domains of bias in randomised controlled trials Limitation Explanation Selection bias (sequence generation and allocation concealment) If those enrolling patients are aware of the group to which the next enrolled patient will be allocated, either because of a non-random sequence that is predictable, or because a truly random sequence was not concealed from the researcher, this may translate into systematic selection bias. This may occur if the researcher chooses not to recruit a participant into that specific group because of: knowledge of that participant s likely prognostic characteristics, and a desire for one group to do better than the other. Performance and Patients, caregivers, those adjudicating or recording outcomes, and data analysts Chronic Heart Failure Methods NICE 2018. All rights reserved. Subject to Notice of rights. ISBN: 978-1-4731-3093-7 Limitation Explanation detection bias (lack of blinding of patients and healthcare professionals) should not be aware of the arm to which patients are allocated. Knowledge of the group can influence: the experience of the placebo effect performance in outcome measures the level of care and attention received, and the methods of measurement or analysis all of which can contribute to systematic bias. Attrition bias Attrition bias results from an unaccounted for loss of data beyond a certain level (a differential of 10% between groups). Loss of data can occur when participants are compulsorily withdrawn from a group by the researchers (for example, when a per- protocol approach is used) or when participants do not attend assessment sessions. 