A new cream was developed to reduce the irritation caused by poison ivy. To test the effectiveness, researchers put an ad online asking for volunteers to participate in the study. One hundred subjects replied, and they were informed that one group would receive the new cream and the other group would receive a cream with no active ingredient. All 100 subjects were exposed to poison ivy. Fifty were then assigned to the group with the new cream, and 50 were assigned to the group with the cream with no active ingredient. After three days, the subjects’ level of irritation was measured. The technicians who surveyed the subjects’ level of irritation as well as the subjects did not know which group received which treatment.
This is a double-blind study because both the subjects and the technicians measuring irritation did not know who received the active cream. Keeping both groups unaware helps reduce bias in how participants report symptoms and how technicians assess them. The cream with no active ingredient is a placebo, but “placebo effect” describes a possible response to it, not the study design.
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