Section 5.2
Designing Experiments
AP Statistics
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Terminology
The individuals on which the experiment is
done are the experimental units.
 When the units are human beings, they
are called subjects.
 A specific experimental condition applied
to the units is called the treatment.

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Experiments
Units
Treatment
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Observe Response
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More terminology
The explanatory variables in an
experiment are often called factors.
 Each treatment is formed by combining a
specific value (often called a level) of each
of the factors.

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The Physicians’ Health Study
Does regularly taking aspirin help protect people
against heart attacks? The Physicians’ Health
Study looked at the effects of two drugs: aspirin
and beta carotene. The body converts beta
carotene into vitamin A, which may help prevent
some forms of cancer. A combination of the
drugs were given to 21,996 male physicians.
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The Physicians’ Health Study

Subjects?
 Physicians

Treatments?
4

(the groups->
Factors?
2
(aspirin & beta carotene)
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The Placebo Effect
“Gastric freezing” is a clever treatment for ulcers in the
upper intestine. The patient swallow a deflated balloon
with tubes attached, the a refrigerated liquid is pumped
through the balloon for an hour. The idea is that cooling
the stomach will reduce its production of acid and so
relieve ulcers.
An experiment reported in the Journal of the American
Medical Association showed that gastric freezing did
reduce the acid production of and relieve ulcer pain.
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The Placebo Effect
The “Gastric freezing” experiment was poorly
designed. The patients’ response may have
been due the placebo effect. A placebo is a
dummy treatment. Many patients respond
favorably to any treatment, even a placebo.
This may be due to trust in the doctor and
expectations of a cure, or simply to the fact that
medical conditions often improve without
treatment.
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Placebo Use
People who receive the placebo are
members of the control group.
 People who receive the “real” treatment
are the treatment group.

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Experiments
Units
Group 1
Aspirin/Beta Carotene
Group 2
Aspirin/Placebo
Compare
Group 3
Placebo/Beta Carotene
Response
Group 4
Placebo/Placebo
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Principles of Experiment Design
Control the effects of lurking variables on
the response, most simply comparing two
or more groups.
 Randomize-use impersonal chance to
assign experimental units treatments
 Replicate each treatment on many units to
reduce chance variation in the results.
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Statistical Significance

An observed effect so large that it would
rarely occur by chance is called
statistically significant.
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Double-Blind

In a double-blind experiment, neither the
subjects nor the people who have contact
with know which treatment a subject
received.
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Experiments without placebos

Matched pair design
 In
a matched pair design, subjects are paired
by matching common important attributes.
 Often the results are a pre-test and post-test
with the unit being “matched” to itself.
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Block Design

A block is a group of experimental units or
subjects that are known before the
experiment to be similar in some way that
is expected to affect the response to the
treatments. In a block design, the random
assignment of units to treatments is
carried out separately within each block.
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Assignment

Exercises: 5.31-5.57 odd
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