For example, osteoarthritis trials tend to use patient-reported pain and function outcomes. Trials with subjective outcomes (those based on feelings such as pain, rather than objectively measured outcomes such as diagnostic records) might be more at risk of bias from lack of blinding. Objective outcomes, such as death are less at risk of bias from lack of blinding, particularly outcome assessors. Studies that did not report double-blinding showed on average odds ratios that were 17% higher than studies that did not. Quantitative evidence demonstrates that blinding in clinical trials affects the results reported: Schulz and coworkers analysed data from 250 randomized controlled trials which had been included in 33 meta-analyses. The participants could determine whether they were taking the intervention pill or the placebo pill because the two types of pill did not match in taste: ExampleĪ well-documented example of lack of blinding is an early study of vitamin C in the prevention of the common cold. For example, for a participant in a clinical trial, not knowing their allocation could make it more difficult to receive individualised treatment. There are arguments that challenge the ethics of blinding study participants. The ethical argument supporting blinding is that whilst the participant loses the knowledge of their treatment, clinical equipoise makes this acceptable. Also, treatment effects or associated adverse events might be specific enough to identify the allocation to a certain intervention. However, for some intervention studies, blinding is not achievable: for instance, it is not possible to blind dietary intakes in a free-living experiment it can be more difficult to achieve blinding in trials of procedures such as surgery. In most studies, blinding should be maintained for the duration of the study until data analysis is complete. To prevent clinicians from providing different treatment for intervention and control groups, and to prevent them from reflecting their views on the allocation to the participants.
To ensure the data analysis is not influenced during or after the trial until analyses are complete for example, by conscious or unconscious selection of statistical tests and reporting.
The impact of blinding on the results of a randomized, placebo-controlled multiple sclerosis clinical trial.
Noseworthy JH, Ebers GC, Vandervoort MK, Farquhar RE, Yetisir E, Roberts R. In a trial of treatments for multiple sclerosis, when blinded neurologists performed disease assessment, there was no difference between intervention and placebo, but when unblinded neurologists performed the assessment, there was an apparent benefit of the intervention over the control treatment.
Outcomes could be assessed differently if the assessors know which participant was receiving which intervention. Study staff collecting data might record differently for different participants if they know which group they are in. Blinding in randomised trials: hiding who got what. They also might be more likely to be lost to follow-up. A participant who knows they are receiving the placebo might be disappointed: they might attend more doctor’s appointments in order to try to get additional treatment. To ensure participants don’t change their behaviour as a result of knowing what their group assignment is and do not report their subjective outcome measures differently as a result. Blinding in a trial can be single, double-blind or triple blind, however, what is important is defining who was blinded as blinding terms are often easily confused. The aim of blinding is to reduce bias due to the knowledge of which intervention or control is being received by study participants.