Blinding

What is blinding?
toc Blinding of key personnel in a trial (participants, investigators, outcome assessors) to the study group assignments can help to prevent performance and detection bias. Performance bias occurs when there are "systematic differences between groups in the care that is provided, or in the exposure to factors other than the interventions of interest;" detection bias occurs when there are "systematic differences between groups in how outcomes are determined" Higgins].

Blinding of participants and study personnel is often addressed by using a placebo or a sham treatment as a control. Making the control group indistinguishable from the treatment group(s) ensures that study personnel won't treat the groups differently (e.g., administering co-interventions) and that participants won't change their behaviour (e.g., dropping out of the study because they are aware that they are in the control group).

Sometimes a trial cannot be blinded (e.g., many surgical trials) or the outcome being assessed is subjective (e.g., pain). In cases like these, it is especially important to consider blinding the outcome assessment so that the individual measuring the outcome is unaware of the group to which the participant was assigned. This will minimize the chance that conscious or unconscious differences in detection will occur between groups.

Tools

 * Appropriate means of blinding study participants, personnel, and outcome assessors:**
 * No blinding or incomplete blinding, but the outcome is not likely to be influenced by lack of blinding
 * Blinding of key personnel ensured, and unlikely that blinding could have been broken

Practical tips for blinding surgical trials: Karanicolas et al. 2010

Recommendations for blinding behavioural interventions: Friedberg et al. 2010 PubMed]

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Examples
On this page we've compiled a number of examples of risk of bias assessments - the good, the bad, and those that are a bit unclear. Feel free to work through them yourself and come up with an assessment of **low **, **unclear ** , or **high ** risk of bias (our judgments and rationale are on the assessments page ), or download a spreadsheet file with the same information. RoB assessments are divided up into the seven major domains: sequence generation, allocation concelment, blinding of participants/personnel, blinding of outcome assessors, incomplete outcome data, selective outcome reporting, and other sources of bias. A quotation is given with the article title following in brackets.

If you have other examples, please add them to the list!

Risk of Bias Guidelines

Download examples:

Blinding of participants/personnel
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 * IV: intravenous; NEC: necrotizing enterocolitis; VLBW: very low birth weight; full citation: A controlled trial of intravenous immune globulin to reduce nosocomial infections in very-low-birth-weight infants

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Blinding of outcome assessors
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