What is a double-blind study?
In experimental research, subjects are randomly assigned to either a treatment or control group. A double-blind study withholds each subject’s group assignment from both the participant and the researcher performing the experiment.
If participants know which group they are assigned to, there is a risk that they might change their behavior in a way that would influence the results. If researchers know which group a participant is assigned to, they might act in a way that reveals the assignment or directly influences the results.
Double blinding guards against these risks, ensuring that any difference between the groups can be attributed to the treatment.
Different types of blinding
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.
If participants know whether they were assigned to the treatment or control group, they might modify their behavior as a result, potentially changing their eventual outcome.
In a single-blind experiment, participants do not know which group they have been placed in until after the experiment has finished.
When the researchers administering the experimental treatment are aware of each participant’s group assignment, they may inadvertently treat those in the control group differently from those in the treatment group. This could reveal to participants their group assignment, or even directly influence the outcome itself.
In double-blind experiments, the group assignment is hidden from both the participant and the person administering the experiment.
Although rarely implemented, triple-blind studies occur when group assignment is hidden not only from participants and administrators, but also from those tasked with analyzing the data after the experiment has concluded.
Researchers may expect a certain outcome and analyze the data in different ways until they arrive at the outcome they expected, even if it is merely a result of chance.
Importance of blinding
Blinding helps ensure a study’s internal validity, or the extent to which you can be confident any link you find in your study is a true cause-and-effect relationship.
Since non-blinded studies can result in participants modifying their behavior or researchers finding effects that do not really exist, blinding is an important tool to avoid bias in all types of scientific research.
Risk of unblinding
Unblinding occurs when researchers have blinded participants or experimenters, but they become aware of who received which treatment before the experiment has ended.
This may result in the same outcomes as would have occurred without any blinding.
Inability to blind
Double or triple blinding is often not possible. While medical experiments can usually use a placebo or fake treatment for blinding, in other types of research, the treatment sometimes cannot be disguised from either the participant or the experimenter. For example, many treatments that physical therapists perform cannot be faked.
In such cases, you must rely on other methods to reduce bias.
- Running a single rather than double- or triple-blind study. Sometimes, although you might not be able to hide what each subject receives, you can still prevent them from knowing whether they are in the treatment or control group. Single blinding is particularly useful in non-medical studies where you cannot use a placebo in the control group.
- Relying on objective measures that participants and experimenters have less control over rather than subjective ones, like measuring fever rather than self-reported pain. This should reduce the possibility that participants or experimenters could influence the results.
- Pre-registering data analysis techniques. This will prevent researchers from trying different measures of analysis until they arrive at the answer they’re expecting.
Frequently asked questions about double-blind studies
- What is blinding?
- What is the difference between single-blind, double-blind and triple-blind studies?
- In a single-blind study, only the participants are blinded.
- In a double-blind study, both participants and experimenters are blinded.
- In a triple-blind study, the assignment is hidden not only from participants and experimenters, but also from the researchers analyzing the data.
- Why is blinding important?
Blinding is important to reduce bias and ensure a study’s internal validity.
If participants know whether they are in a control or treatment group, they may adjust their behavior in ways that affect the outcome that researchers are trying to measure. If the people administering the treatment are aware of group assignment, they may treat participants differently and thus directly or indirectly influence the final results.