Step three - continued

Performance bias

This refers to the systematic differences in the care given to the participants in the comparison groups, other than the intervention under investigation. Therefore to protect against unintended differences in care and placebo effects, those providing and receiving care may be 'blinded' so they are unaware of the group to which the receiver of care has been allocated. There is good evidence that those who are aware of their assignment status report more symptoms, which may lead to biased results.

Detection bias

Occurs if the knowledge of patient assignment influences the process of outcome assessment. In order to get around this, outcome assessors and patients may be blinded.

Attrition bias

Attrition bias may be introduced when protocol deviations and loss to follow-up leads to the exclusion of patients after they have been allocated to treatment groups.