Double-Blind Experiment — Definition, Formula & Examples
A double-blind experiment is one in which neither the subjects nor the researchers who interact with them know which treatment each subject is receiving. This prevents both groups from unconsciously influencing the results.
In a double-blind experimental design, knowledge of treatment allocation is concealed from both the experimental units and the individuals administering or measuring the treatments, thereby controlling for subject-expectation bias (placebo effect) and observer bias simultaneously.
How It Works
A third party assigns treatments using random assignment and keeps a coded master list. Subjects receive either the actual treatment or a placebo that looks identical, so they cannot tell which group they belong to. The researchers collecting and recording data also do not know each subject's group assignment. Only after data collection is complete does the third party reveal the assignments for analysis. This two-layer concealment ensures that neither conscious nor unconscious expectations distort the response variable.
Example
Problem: A pharmaceutical company tests whether a new pill reduces headache duration. They recruit 200 volunteers. Describe how to set up a double-blind experiment.
Step 1: Use random assignment to place the 200 volunteers into two groups of 100. One group receives the new pill (treatment); the other receives a sugar pill identical in size, shape, and color (placebo).
Step 2: A pharmacist who has no other role in the study labels each pill bottle with only a code number. Neither the subjects nor the doctors measuring headache duration know which bottles contain the real pill.
Step 3: After all headache durations are recorded, the pharmacist reveals the code. The researchers then compare the mean headache duration of the treatment group to that of the placebo group using a hypothesis test.
Answer: Because both subjects and researchers are blind to the assignments, any observed difference in headache duration can be more confidently attributed to the pill itself rather than to bias.
Why It Matters
On the AP Statistics exam, free-response questions on experimental design frequently ask you to explain how blinding reduces bias. Describing a double-blind setup—and distinguishing it from a single-blind design—can earn you full credit on these questions. In careers like clinical research and psychology, double-blinding is often the ethical and scientific standard for drawing causal conclusions.
Common Mistakes
Mistake: Confusing double-blind with single-blind. Students sometimes say an experiment is double-blind when only the subjects are unaware of their treatment.
Correction: Double-blind means both the subjects AND the people measuring or administering the treatment are unaware. If only one side is blind, it is single-blind.