Section 5.2Designing Experiments

AP Statistics

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AP Statistics, Section 5.1, Part 1 2

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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Units Treatment Observe Response

Experiments

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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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Aspirin/Beta CaroteneGroup 1

UnitsCompare

Response

Experiments

Aspirin/PlaceboGroup 2

Placebo/Beta Carotene

Group 3

Placebo/PlaceboGroup 4

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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