Control Groups, Placebos & Blind Experiments (College Board AP® Statistics): Study Guide
Syllabus Edition
First teaching 2026
First exams 2027
Control groups & placebos
What is a control group?
A control group is a treatment group in an experiment that are purposefully given either
an inactive form of the treatment
or the pre-existing (baseline) treatment
Having a control group makes it easier to determine whether the treatment of interest (applied to another group) has any effect
Examiner Tips and Tricks
It is not correct in the exam to say that a control group receive 'no treatment', because being in a control group is, in itself, a type of treatment!
What is a placebo?
A placebo is a dummy (inactive) treatment that the experimental units believe is an active treatment
e.g. telling individuals they are taking a pain-relief drink when in fact it is water
Control groups can use placebos as inactive forms of treatment
though placebos are not always required
e.g. when comparing a well-established old treatment to a new and untested treatment
What is the placebo effect?
In some cases, experimental units may show a response to a placebo
This type of response is called the placebo effect
e.g. feeling like you are smarter after taking what you thought was an intelligence pill (which turned out to be just a sugar pill)
Single & double-blind experiments
What is a single-blind experiment?
A single-blind experiment is one in which the participants do not know which treatment they are receiving
e.g. participants do not know if they have a placebo pill or a real pill
It helps to reduce the placebo effect
as it stops participants from changing their behavior based on knowing what treatment they are having
The researchers conducting the experiment know which participants received which treatments
What is a double-blind experiment?
A double-blind experiment is one in which participants and the researchers (in particular those interacting with participants and measuring their responses) do not know which groups are assigned which treatments
This helps to reduce the placebo effect
and also helps to reduce any bias introduced by the researchers themselves (consciously or unconsciously)
There will be an experimental designer (who is not a participant, nor a researcher measuring the responses of the participants) who will know which participants received which treatments
After the experiment has been conducted, the results are revealed
Worked Example
A sports nutritionist is evaluating a new sports drink designed to improve endurance. Volunteer athletes are randomly assigned to receive either the new sports drink or a standard sports drink. Both drinks look and taste exactly the same. The athletes are not told which drink they are consuming. Furthermore, the trainers who hand out the drinks to the athletes and record their treadmill endurance times are also not told which drink each athlete receives.
Which of the following best describes the blinding in this experiment?
(A) Single-blind, because the athletes do not know which treatment they are receiving.
(B) Single-blind, because the trainers do not know which treatment the athletes are receiving.
(C) Double-blind, because neither the athletes nor the trainers interacting with them know which treatment is being administered.
(D) Double-blind, because the athletes do not know the treatment and they were randomly assigned to the two groups.
Answer:
In a single-blind experiment, either the participants do not know which treatment they are receiving but the researchers interacting with them do, or vice versa
In a double-blind experiment, neither the participants nor the members of the research team who interact with them know which treatment each participant is receiving
In this scenario, because both the athletes (the participants) and the trainers (the research team members directly interacting with them and recording the response variable) are unaware of which sports drink is being administered, the experiment is double-blind
The correct answer is C
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