Blinding in Clinical Trials

(This section is organised in the form of a book, please follow the blue arrows to navigate through the book or by following the navigation panel on the right side of the page.)

• The trial participant
  Blinding participants to the treatment they have received is particularly important when the response criteria are subjective, such as alleviation of pain, but less important for objective criteria, such as disease progression (e.g. cancer). If participants are not blinded, knowledge of group assignment may affect their behaviour in the trial and their responses to subjective outcome measures. For example, a participant who is aware that he is not receiving active treatment may be less likely to comply with the trial protocol. Those aware that they are receiving or not receiving therapy are more likely to provide biased assessments of the effectiveness of the intervention — most likely in opposite directions — than blinded participants.
  
  
• Clinical staff administering treatment
  Similarly, medical staff and clinicians caring for patients should be blinded to treatment allocation to minimise possible bias in patient management and in assessing disease status. Blinded clinicians are much less likely to transfer their attitudes to participants or to provide differential treatment to the active and control (placebo) groups than are unblinded clinicians.
  
  
• The doctor assessing treatment
  Blinding of data collectors and outcome adjudicators (sometimes the same individuals) is crucial to ensure unbiased ascertainment of outcomes. For example, in a randomised controlled trial in patients, neither active treatment regimen (tested medication vs. comparator) was superior to placebo when assessed by blinded specialists, but there was an apparent benefit of treatment with the test medication, when unblinded specialists performed the assessments.
  
  
• The team interpreting results
  Bias may also be introduced during the statistical analysis of the trial through the selective use and reporting of statistical tests. This may be a subconscious process spurred by investigators eager to see a positive result, but the consequences are profound. The best method to avoid this potential bias is blinding of the data analyst until the entire analysis has been completed.

• For surgical procedures*.
  
• When changes in lifestyle are required.
  
• When endpoints are objective and cannot be interpreted in different ways.
  
• For studies in life-threatening situations.
  
• In post-marketing surveillance.
  
• When ethical considerations do not permit blinding.
• When no control group can be used. 

• Considered best-controlled trial design.
  
• Decreased chance of preconceived notions or physical cues (e.g. the placebo effect, observer bias, experimenter's bias) to distort the results.
  
• The key that identifies the participants and which group they belonged to is kept by a third party, and is not revealed to the researchers until the study is over.
• Should be used whenever possible.
  

• Medicine may still be labelled as A or B during analysis.
• Analyst is blinded to which treatment is which.
  
• Helps to avoid bias in the analysed results.

• To make clinical treatment decisions or when an unexpected serious adverse event occurs and the intervention must be made known. This is called emergency unblinding (to protect the participant). Unblinding should only occur for that participant and not the entire study.
  
• During an unmasked analysis in accordance with the study analysis plan (e.g. a planned interim analysis).
  
• At the request of the Data Safety Monitoring Board.
  
• At the conclusion of the study to determine the effect of the intervention.

1. Contacting the holder of the blinding information (e.g. sponsor, CRO, etc.) to find out details of what treatment a participant received.
   
2. Contacting an automated service.