A randomized controlled trial: Different types of blinding

T. Dhasaratharaman

Statistician, Kauvery Hospital, Cantonment, Trichy

Background

Blinding means withholding which group each participant has been assigned to. Studies may use Single-, Double- or Triple-blinding.

Single-blinding occurs in many different kinds of studies, but double- and triple-blinding are mainly used in medical research.

A trial can be “open labeled” or “blinded”. By the process of blinding, we make the participant and/or assessing physician unaware of the treatment he/she is going to receive. Thus, the element of bias which can creep in owing to personal preference or subjective component to the assessment of outcome (e.g., a tool like physician global score is used to assess the outcome) can be eliminated. The process has now been further extended to include the statistician analyzing the result to make it fool proof. Thus, blinding is helpful in eliminating intentional or unintentional bias, increasing the objectivity of results, and ensuring the credibility of study conclusions.

Types of Blinding

Open-labeled or unblinded: All parties involved in a study are aware of the treatment the participants are receiving. Although blinding is desirable, sometimes it may not be possible or feasible. This type of study design suffers from low credibility but may be acceptable if endpoints are indisputably objective (e.g., survival or death)

Example: You are studying the impact of a new school instruction program aimed at improving students’ reading comprehension skills.

You randomly assign some students to the new program (the treatment group), while others are instructed with a standard program (the control group). You use single blinding: you do not inform students whether they are receiving the new instruction program or the standard one.

If students become aware of which program they have been assigned to – for example, by talking to previous students about the content of the program – they may change their behavior. Students in the control group might work harder on their reading skills to make up for not receiving the new program, or conversely to put in less effort instead since they might believe the other students will do better than them anyway.

Thus, the results of your study could be invalid unless you prevent any unblinding.

Single-blind: The participants in a study might drop out from study or might give false assessment if they come to know that they are receiving “no treatment.” In addition, they might develop a placebo effect, if they know they are receiving “new treatment.” All these biases can be eliminated by single-blinding. In this, a group of individuals (usually the participants) do not know the intervention he or she is going to receive. Conventionally, it refers to participant-blinded but logically the group of individuals blinded can also be the outcome assessor. Thus, a single-blind trial can be either participant-blind or assessor-blind, and it is better to specify who is blinded, instead of saying single-blind

Example: You have developed a new flu vaccine. In order to test the effectiveness of your new treatment, you run an experiment, giving half of your participants the flu vaccine and the other half a fake vaccine that will have no effect (to control for the placebo effect).

If participants in the control group realize they have received a fake vaccine and are not protected against the flu, they might modify their behavior in ways that lower their chances of becoming sick – frequently washing their hands, avoiding crowded areas, etc. This behavior could narrow the gap in sickness rates between the control group and the treatment group, thus making the vaccine seem less effective than it really is.

To prevent such an outcome, in a single-blind study, you hide from the participants which vaccine – real or fake – each of them received.

Double-blind: Like participants, the investigator/observer may influence the results of the study, if they are aware if a group of individuals are receiving a particular treatment. For example, if the endpoint is subjective (e.g. physician global scale), they might record a more favorable response for treatment of their preference. In addition, they might influence participants’ assessment of a particular treatment during follow-up meetings. In double-blinding, neither the participant nor the investigator/observer/outcome assessor is aware of the treatment allotted. The investigator is the person carrying out the research. The observer or the outcome assessor is the person who assesses the parameters of the study.

Example: In the flu vaccine study that you are running, you have recruited several experimenters to administer your vaccine and measure the outcomes of your participants.

If these experimenters knew which vaccines were real and which were fake, they might accidentally reveal this information to the participants, thus influencing their behavior and indirectly the results.

They could even directly influence the results. For instance, if experimenters expect the vaccine to result in lower levels of flu symptoms, they might accidentally measure symptoms incorrectly, thus making the vaccine appear more effective than it really is.

To avoid this, you hide group assignments from both the participants and the experimenters giving the vaccines – a double-blind study.

Triple-blind: Triple-blinding is done to eliminate the bias of data analysts. In triple blinding, the participant, investigator, and the data analyst are unaware of the treatment given.

Example: In your vaccine study, you have also recruited assistants to analyze the data you gathered on flu infection rates. You decide to hide the group assignments from the participants, the people administering the experiment, and the people analyzing the data – a triple-blind study.

To achieve triple blinding, you assign each participant to group 1 or group 2, but do not inform the data analysts which number represents which group.

However, instead of expressing whether the trial is single, double, or triple blinded, it is more pertinent to specify who exactly is going to be blinded.

 

Mr. T. Dhasaratharaman
Assistant Manager – Statistician

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